diff --git a/Jakefile.js b/Jakefile.js
index c85d1b40..4340aa5e 100644
--- a/Jakefile.js
+++ b/Jakefile.js
@@ -64,8 +64,8 @@ task('build', {async: true}, function () {
       './src/DataSet.js',
       './src/DataView.js',
 
+      './src/timeline/stack.js',
       './src/timeline/TimeStep.js',
-      './src/timeline/Stack.js',
       './src/timeline/Range.js',
       './src/timeline/component/Component.js',
       './src/timeline/component/Panel.js',
@@ -76,7 +76,6 @@ task('build', {async: true}, function () {
       './src/timeline/component/ItemSet.js',
       './src/timeline/component/item/*.js',
       './src/timeline/component/Group.js',
-      './src/timeline/component/GroupSet.js',
       './src/timeline/Timeline.js',
 
       './src/graph/dotparser.js',
diff --git a/src/module/exports.js b/src/module/exports.js
index a2f25d6e..7c2cd456 100644
--- a/src/module/exports.js
+++ b/src/module/exports.js
@@ -7,7 +7,7 @@ var vis = {
   DataSet: DataSet,
   DataView: DataView,
   Range: Range,
-  Stack: Stack,
+  stack: stack,
   TimeStep: TimeStep,
 
   components: {
diff --git a/src/timeline/Stack.js b/src/timeline/Stack.js
deleted file mode 100644
index 0e249c12..00000000
--- a/src/timeline/Stack.js
+++ /dev/null
@@ -1,168 +0,0 @@
-// TODO: turn Stack into a Mixin?
-
-/**
- * @constructor Stack
- * Stacks items on top of each other.
- * @param {Object} [options]
- */
-function Stack (options) {
-  this.options = options || {};
-  this.defaultOptions = {
-    order: function (a, b) {
-      // Order: ranges over non-ranges, ranged ordered by width,
-      //        and non-ranges ordered by start.
-      if (a instanceof ItemRange) {
-        if (b instanceof ItemRange) {
-          var aInt = (a.data.end - a.data.start);
-          var bInt = (b.data.end - b.data.start);
-          return (aInt - bInt) || (a.data.start - b.data.start);
-        }
-        else {
-          return -1;
-        }
-      }
-      else {
-        if (b instanceof ItemRange) {
-          return 1;
-        }
-        else {
-          return (a.data.start - b.data.start);
-        }
-      }
-    },
-    margin: {
-      item: 10,
-      axis: 20
-    }
-  };
-}
-
-/**
- * Set options for the stack
- * @param {Object} options  Available options:
- *                          {Number} [margin.item=10]
- *                          {Number} [margin.axis=20]
- *                          {function} [order]  Stacking order
- */
-Stack.prototype.setOptions = function setOptions (options) {
-  util.extend(this.options, options);
-};
-
-/**
- * Order an array with items using a predefined order function for items
- * @param {Item[]} items
- */
-Stack.prototype.order = function order(items) {
-  //order the items
-  var order = this.options.order || this.defaultOptions.order;
-  if (!(typeof order === 'function')) {
-    throw new Error('Option order must be a function');
-  }
-  items.sort(order);
-};
-
-/**
- * Order items by their start data
- * @param {Item[]} items
- */
-Stack.prototype.orderByStart = function orderByStart(items) {
-  items.sort(function (a, b) {
-    return a.data.start - b.data.start;
-  });
-};
-
-/**
- * Order items by their end date. If they have no end date, their start date
- * is used.
- * @param {Item[]} items
- */
-Stack.prototype.orderByEnd = function orderByEnd(items) {
-  items.sort(function (a, b) {
-    var aTime = ('end' in a.data) ? a.data.end : a.data.start,
-        bTime = ('end' in b.data) ? b.data.end : b.data.start;
-
-    return aTime - bTime;
-  });
-};
-
-/**
- * Adjust vertical positions of the events such that they don't overlap each
- * other.
- * @param {Item[]} items          All visible items
- * @param {boolean} [force=false] If true, all items will be re-stacked.
- *                                If false (default), only items having a
- *                                top===null will be re-stacked
- */
-Stack.prototype.stack = function stack (items, force) {
-  var i,
-      iMax,
-      options = this.options,
-      marginItem,
-      marginAxis;
-
-  if (options.margin && options.margin.item !== undefined) {
-    marginItem = options.margin.item;
-  }
-  else {
-    marginItem = this.defaultOptions.margin.item
-  }
-  if (options.margin && options.margin.axis !== undefined) {
-    marginAxis = options.margin.axis;
-  }
-  else {
-    marginAxis = this.defaultOptions.margin.axis
-  }
-
-  if (force) {
-    // reset top position of all items
-    for (i = 0, iMax = items.length; i < iMax; i++) {
-      items[i].top = null;
-    }
-  }
-
-  // calculate new, non-overlapping positions
-  for (i = 0, iMax = items.length; i < iMax; i++) {
-    var item = items[i];
-    if (item.top === null) {
-      // initialize top position
-      item.top = marginAxis;
-
-      do {
-        // TODO: optimize checking for overlap. when there is a gap without items,
-        //       you only need to check for items from the next item on, not from zero
-        var collidingItem = null;
-        for (var j = 0, jj = items.length; j < jj; j++) {
-          var other = items[j];
-          if (other.top !== null && other !== item && this.collision(item, other, marginItem)) {
-            collidingItem = other;
-            break;
-          }
-        }
-
-        if (collidingItem != null) {
-          // There is a collision. Reposition the event above the colliding element
-          item.top = collidingItem.top + collidingItem.height + marginItem;
-        }
-      } while (collidingItem);
-    }
-  }
-};
-
-/**
- * Test if the two provided items collide
- * The items must have parameters left, width, top, and height.
- * @param {Item} a          The first item
- * @param {Item} b          The second item
- * @param {Number} margin   A minimum required margin.
- *                          If margin is provided, the two items will be
- *                          marked colliding when they overlap or
- *                          when the margin between the two is smaller than
- *                          the requested margin.
- * @return {boolean}        true if a and b collide, else false
- */
-Stack.prototype.collision = function collision (a, b, margin) {
-  return ((a.left - margin) < (b.left + b.width) &&
-      (a.left + a.width + margin) > b.left &&
-      (a.top - margin) < (b.top + b.height) &&
-      (a.top + a.height + margin) > b.top);
-};
diff --git a/src/timeline/component/Group.js b/src/timeline/component/Group.js
index 66e6a19b..408cad52 100644
--- a/src/timeline/component/Group.js
+++ b/src/timeline/component/Group.js
@@ -9,7 +9,12 @@ function Group (groupId, itemSet) {
   this.itemSet = itemSet;
 
   this.dom = {};
-  this.items = {}; // items filtered by groupId of this group
+  this.items = {};        // items filtered by groupId of this group
+  this.visibleItems = []; // items currently visible in window
+  this.orderedItems = {   // items sorted by start and by end
+    byStart: [],
+    byEnd: []
+  };
 
   this._create();
 }
@@ -62,19 +67,62 @@ Group.prototype.getAxis = function getAxis() {
   return this.dom.axis;
 };
 
-/**
- * Get the height of the itemsets background
- * @return {Number} height
- */
-Group.prototype.getBackgroundHeight = function getBackgroundHeight() {
-  return this.itemSet.height;
-};
-
 /**
  * Repaint this group
+ * @param {{start: number, end: number}} range
+ * @param {number | {item: number, axis: number}} margin
+ * @param {boolean} [restack=false]  Force restacking of all items
+ * @return {boolean} Returns true if the group is resized
  */
-Group.prototype.repaint = function repaint() {
-  // TODO: implement Group.repaint
+Group.prototype.repaint = function repaint(range, margin, restack) {
+  if (typeof margin === 'number') {
+    margin = {
+      item: margin,
+      axis: margin
+    };
+  }
+
+  // update visible items
+  this.visibleItems = this._updateVisibleItems(this.orderedItems, this.visibleItems, range);
+
+  // reposition visible items vertically
+  stack.stack(this.visibleItems, margin, restack);
+  this.stackDirty = false;
+  for (var i = 0, ii = this.visibleItems.length; i < ii; i++) {
+    this.visibleItems[i].repositionY();
+  }
+
+  // recalculate the height of the group
+  var height;
+
+  // determine the height from the stacked items
+  var visibleItems = this.visibleItems;
+  if (visibleItems.length) {
+    var min = visibleItems[0].top;
+    var max = visibleItems[0].top + visibleItems[0].height;
+    util.forEach(visibleItems, function (item) {
+      min = Math.min(min, item.top);
+      max = Math.max(max, (item.top + item.height));
+    });
+    height = (max - min) + margin.axis + margin.item;
+  }
+  else {
+    height = margin.axis + margin.item;
+  }
+
+  var resized = (this.height != height);
+
+  // calculate actual size and position
+  var foreground = this.dom.foreground;
+  this.top = foreground.offsetTop;
+  this.left = foreground.offsetLeft;
+  this.width = foreground.offsetWidth;
+  this.height = height;
+
+  // apply new height
+  foreground.style.height  = height + 'px';
+
+  return resized;
 };
 
 /**
@@ -130,6 +178,11 @@ Group.prototype.hide = function hide() {
 Group.prototype.add = function add(item) {
   this.items[item.id] = item;
   item.setParent(this);
+
+  if (item instanceof ItemRange && this.visibleItems.indexOf(item) == -1) {
+    var range = this.itemSet.range; // TODO: not nice accessing the range like this
+    this._checkIfVisible(item, this.visibleItems, range);
+  }
 };
 
 /**
@@ -139,5 +192,210 @@ Group.prototype.add = function add(item) {
 Group.prototype.remove = function remove(item) {
   delete this.items[item.id];
   item.setParent(this.itemSet);
+
+  // remove from visible items
+  var index = this.visibleItems.indexOf(item);
+  if (index != -1) this.visibleItems.splice(index, 1);
+
+  // TODO: also remove from ordered items?
+};
+
+/**
+ * Order the items
+ * @private
+ */
+Group.prototype._order = function _order() {
+  var array = util.toArray(this.items);
+  this.orderedItems.byStart = array;
+  this.orderedItems.byEnd = this._constructByEndArray(array);
+
+  // reorder the items
+  stack.orderByStart(this.orderedItems.byStart);
+  stack.orderByEnd(this.orderedItems.byEnd);
 };
 
+/**
+ * Create an array containing all items being a range (having an end date)
+ * @param {Item[]} array
+ * @returns {ItemRange[]}
+ * @private
+ */
+Group.prototype._constructByEndArray = function _constructByEndArray(array) {
+  var endArray = [];
+
+  for (var i = 0; i < array.length; i++) {
+    if (array[i] instanceof ItemRange) {
+      endArray.push(array[i]);
+    }
+  }
+  return endArray;
+};
+
+/**
+ * Update the visible items
+ * @param {{byStart: Item[], byEnd: Item[]}} orderedItems   All items ordered by start date and by end date
+ * @param {Item[]} visibleItems                             The previously visible items.
+ * @param {{start: number, end: number}} range              Visible range
+ * @return {Item[]} visibleItems                            The new visible items.
+ * @private
+ */
+Group.prototype._updateVisibleItems = function _updateVisibleItems(orderedItems, visibleItems, range) {
+  var initialPosByStart,
+      newVisibleItems = [],
+      i;
+
+  // first check if the items that were in view previously are still in view.
+  // this handles the case for the ItemRange that is both before and after the current one.
+  if (visibleItems.length > 0) {
+    for (i = 0; i < visibleItems.length; i++) {
+      this._checkIfVisible(visibleItems[i], newVisibleItems, range);
+    }
+  }
+
+  // If there were no visible items previously, use binarySearch to find a visible ItemPoint or ItemRange (based on startTime)
+  if (newVisibleItems.length == 0) {
+    initialPosByStart = this._binarySearch(orderedItems, range, false);
+  }
+  else {
+    initialPosByStart = orderedItems.byStart.indexOf(newVisibleItems[0]);
+  }
+
+  // use visible search to find a visible ItemRange (only based on endTime)
+  var initialPosByEnd = this._binarySearch(orderedItems, range, true);
+
+  // if we found a initial ID to use, trace it up and down until we meet an invisible item.
+  if (initialPosByStart != -1) {
+    for (i = initialPosByStart; i >= 0; i--) {
+      if (this._checkIfInvisible(orderedItems.byStart[i], newVisibleItems, range)) {break;}
+    }
+    for (i = initialPosByStart + 1; i < orderedItems.byStart.length; i++) {
+      if (this._checkIfInvisible(orderedItems.byStart[i], newVisibleItems, range)) {break;}
+    }
+  }
+
+  // if we found a initial ID to use, trace it up and down until we meet an invisible item.
+  if (initialPosByEnd != -1) {
+    for (i = initialPosByEnd; i >= 0; i--) {
+      if (this._checkIfInvisible(orderedItems.byEnd[i], newVisibleItems, range)) {break;}
+    }
+    for (i = initialPosByEnd + 1; i < orderedItems.byEnd.length; i++) {
+      if (this._checkIfInvisible(orderedItems.byEnd[i], newVisibleItems, range)) {break;}
+    }
+  }
+
+  return newVisibleItems;
+};
+
+/**
+ * This function does a binary search for a visible item. The user can select either the this.orderedItems.byStart or .byEnd
+ * arrays. This is done by giving a boolean value true if you want to use the byEnd.
+ * This is done to be able to select the correct if statement (we do not want to check if an item is visible, we want to check
+ * if the time we selected (start or end) is within the current range).
+ *
+ * The trick is that every interval has to either enter the screen at the initial load or by dragging. The case of the ItemRange that is
+ * before and after the current range is handled by simply checking if it was in view before and if it is again. For all the rest,
+ * either the start OR end time has to be in the range.
+ *
+ * @param {{byStart: Item[], byEnd: Item[]}} orderedItems
+ * @param {{start: number, end: number}} range
+ * @param {Boolean} byEnd
+ * @returns {number}
+ * @private
+ */
+Group.prototype._binarySearch = function _binarySearch(orderedItems, range, byEnd) {
+  var array = [];
+  var byTime = byEnd ? 'end' : 'start';
+  if (byEnd == true) {array = orderedItems.byEnd;  }
+  else               {array = orderedItems.byStart;}
+
+  var interval = range.end - range.start;
+
+  var found = false;
+  var low = 0;
+  var high = array.length;
+  var guess = Math.floor(0.5*(high+low));
+  var newGuess;
+
+  if (high == 0) {guess = -1;}
+  else if (high == 1) {
+    if ((array[guess].data[byTime] > range.start - interval) && (array[guess].data[byTime] < range.end)) {
+      guess =  0;
+    }
+    else {
+      guess = -1;
+    }
+  }
+  else {
+    high -= 1;
+    while (found == false) {
+      if ((array[guess].data[byTime] > range.start - interval) && (array[guess].data[byTime] < range.end)) {
+        found = true;
+      }
+      else {
+        if (array[guess].data[byTime] < range.start - interval) { // it is too small --> increase low
+          low = Math.floor(0.5*(high+low));
+        }
+        else {  // it is too big --> decrease high
+          high = Math.floor(0.5*(high+low));
+        }
+        newGuess = Math.floor(0.5*(high+low));
+        // not in list;
+        if (guess == newGuess) {
+          guess = -1;
+          found = true;
+        }
+        else {
+          guess = newGuess;
+        }
+      }
+    }
+  }
+  return guess;
+};
+
+/**
+ * this function checks if an item is invisible. If it is NOT we make it visible
+ * and add it to the global visible items. If it is, return true.
+ *
+ * @param {Item} item
+ * @param {Item[]} visibleItems
+ * @param {{start:number, end:number}} range
+ * @returns {boolean}
+ * @private
+ */
+Group.prototype._checkIfInvisible = function _checkIfInvisible(item, visibleItems, range) {
+  if (item.isVisible(range)) {
+    if (!item.displayed) item.show();
+    item.repositionX();
+    if (visibleItems.indexOf(item) == -1) {
+      visibleItems.push(item);
+    }
+    return false;
+  }
+  else {
+    return true;
+  }
+};
+
+/**
+ * this function is very similar to the _checkIfInvisible() but it does not
+ * return booleans, hides the item if it should not be seen and always adds to
+ * the visibleItems.
+ * this one is for brute forcing and hiding.
+ *
+ * @param {Item} item
+ * @param {Array} visibleItems
+ * @param {{start:number, end:number}} range
+ * @private
+ */
+Group.prototype._checkIfVisible = function _checkIfVisible(item, visibleItems, range) {
+  if (item.isVisible(range)) {
+    if (!item.displayed) item.show();
+    // reposition item horizontally
+    item.repositionX();
+    visibleItems.push(item);
+  }
+  else {
+    if (item.displayed) item.hide();
+  }
+};
diff --git a/src/timeline/component/GroupSet.js b/src/timeline/component/GroupSet.js
index 1e94fa3c..387324da 100644
--- a/src/timeline/component/GroupSet.js
+++ b/src/timeline/component/GroupSet.js
@@ -1,3 +1,5 @@
+// TODO: remove groupset
+
 /**
  * An GroupSet holds a set of groups
  * @param {Panel} contentPanel      Panel where the ItemSets will be created
diff --git a/src/timeline/component/ItemSet.js b/src/timeline/component/ItemSet.js
index 349e93aa..a9f2445a 100644
--- a/src/timeline/component/ItemSet.js
+++ b/src/timeline/component/ItemSet.js
@@ -61,18 +61,11 @@ function ItemSet(backgroundPanel, axisPanel, labelPanel, options) {
     }
   };
 
-  this.items = {};        // object with an Item for every data item
-  this.orderedItems = {
-    byStart: [],
-    byEnd: []
-  };
-
-  this.groups = {}; // Group object for every group
+  this.items = {};      // object with an Item for every data item
+  this.groups = {};     // Group object for every group
   this.groupIds = [];
 
-  this.visibleItems = []; // visible, ordered items
   this.selection = [];  // list with the ids of all selected nodes
-  this.stack = new Stack(Object.create(this.options));
   this.stackDirty = true; // if true, all items will be restacked on next repaint
 
   this.touchParams = {}; // stores properties while dragging
@@ -269,127 +262,18 @@ ItemSet.prototype.getFrame = function getFrame() {
   return this.frame;
 };
 
-/**
- * This function does a binary search for a visible item. The user can select either the this.orderedItems.byStart or .byEnd
- * arrays. This is done by giving a boolean value true if you want to use the byEnd.
- * This is done to be able to select the correct if statement (we do not want to check if an item is visible, we want to check
- * if the time we selected (start or end) is within the current range).
- *
- * The trick is that every interval has to either enter the screen at the initial load or by dragging. The case of the ItemRange that is
- * before and after the current range is handled by simply checking if it was in view before and if it is again. For all the rest,
- * either the start OR end time has to be in the range.
- *
- * @param {{byStart: Item[], byEnd: Item[]}} orderedItems
- * @param {{start: number, end: number}} range
- * @param {Boolean} byEnd
- * @returns {number}
- * @private
- */
-ItemSet.prototype._binarySearch = function _binarySearch(orderedItems, range, byEnd) {
-  var array = [];
-  var byTime = byEnd ? "end" : "start";
-  if (byEnd == true) {array = orderedItems.byEnd;  }
-  else               {array = orderedItems.byStart;}
-
-  var interval = range.end - range.start;
-
-  var found = false;
-  var low = 0;
-  var high = array.length;
-  var guess = Math.floor(0.5*(high+low));
-  var newGuess;
-
-  if (high == 0) {guess = -1;}
-  else if (high == 1) {
-    if ((array[guess].data[byTime] > range.start - interval) && (array[guess].data[byTime] < range.end)) {
-      guess =  0;
-    }
-    else {
-      guess = -1;
-    }
-  }
-  else {
-    high -= 1;
-    while (found == false) {
-      if ((array[guess].data[byTime] > range.start - interval) && (array[guess].data[byTime] < range.end)) {
-        found = true;
-      }
-      else {
-        if (array[guess].data[byTime] < range.start - interval) { // it is too small --> increase low
-          low = Math.floor(0.5*(high+low));
-        }
-        else {  // it is too big --> decrease high
-          high = Math.floor(0.5*(high+low));
-        }
-        newGuess = Math.floor(0.5*(high+low));
-        // not in list;
-        if (guess == newGuess) {
-          guess = -1;
-          found = true;
-        }
-        else {
-          guess = newGuess;
-        }
-      }
-    }
-  }
-  return guess;
-};
-
-/**
- * this function checks if an item is invisible. If it is NOT we make it visible and add it to the global visible items. If it is, return true.
- *
- * @param {Item} item
- * @param {Item[]} visibleItems
- * @returns {boolean}
- * @private
- */
-ItemSet.prototype._checkIfInvisible = function _checkIfInvisible(item, visibleItems) {
-  if (item.isVisible(this.range)) {
-    if (!item.displayed) item.show();
-    item.repositionX();
-    if (visibleItems.indexOf(item) == -1) {
-      visibleItems.push(item);
-    }
-    return false;
-  }
-  else {
-    return true;
-  }
-};
-
-
-/**
- * this function is very similar to the _checkIfInvisible() but it does not return booleans, hides the item if it should not be seen and always adds to the visibleItems.
- * this one is for brute forcing and hiding.
- *
- * @param {Item} item
- * @param {Array} visibleItems
- * @private
- */
-ItemSet.prototype._checkIfVisible = function _checkIfVisible(item, visibleItems) {
-  if (item.isVisible(this.range)) {
-    if (!item.displayed) item.show();
-    // reposition item horizontally
-    item.repositionX();
-    visibleItems.push(item);
-  }
-  else {
-    if (item.displayed) item.hide();
-  }
-};
-
 /**
  * Repaint the component
  * @return {boolean} Returns true if the component is resized
  */
 ItemSet.prototype.repaint = function repaint() {
-  var asSize = util.option.asSize,
+  var margin = this.options.margin,
+      range = this.range,
+      asSize = util.option.asSize,
       asString = util.option.asString,
       options = this.options,
       orientation = this.getOption('orientation'),
-      frame = this.frame,
-      i, ii;
+      frame = this.frame;
 
   // update className
   frame.className = 'itemset' + (options.className ? (' ' + asString(options.className)) : '');
@@ -400,36 +284,14 @@ ItemSet.prototype.repaint = function repaint() {
   this.lastVisibleInterval = visibleInterval;
   this.lastWidth = this.width;
 
-  this.visibleItems = this._updateVisibleItems(this.orderedItems, this.visibleItems, this.range);
-
-  // reposition visible items vertically.
-  //this.stack.order(this.visibleItems); // TODO: improve ordering
-  var force = this.stackDirty || zoomed; // force re-stacking of all items if true
-  this.stack.stack(this.visibleItems, force);
+  // repaint all groups
+  var restack = zoomed || this.stackDirty;
+  var height = 0;
+  util.forEach(this.groups, function (group) {
+    group.repaint(range, margin, restack);
+    height += group.height;
+  });
   this.stackDirty = false;
-  for (i = 0, ii = this.visibleItems.length; i < ii; i++) {
-    this.visibleItems[i].repositionY();
-  }
-
-  // recalculate the height of the itemset
-  var marginAxis = (options.margin && 'axis' in options.margin) ? options.margin.axis : this.itemOptions.margin.axis,
-      marginItem = (options.margin && 'item' in options.margin) ? options.margin.item : this.itemOptions.margin.item,
-      height;
-
-  // determine the height from the stacked items
-  var visibleItems = this.visibleItems;
-  if (visibleItems.length) {
-    var min = visibleItems[0].top;
-    var max = visibleItems[0].top + visibleItems[0].height;
-    util.forEach(visibleItems, function (item) {
-      min = Math.min(min, item.top);
-      max = Math.max(max, (item.top + item.height));
-    });
-    height = (max - min) + marginAxis + marginItem;
-  }
-  else {
-    height = marginAxis + marginItem;
-  }
 
   // reposition frame
   frame.style.left    = asSize(options.left, '');
@@ -457,61 +319,6 @@ ItemSet.prototype.repaint = function repaint() {
   return this._isResized();
 };
 
-/**
- * Update the visible items
- * @param {{byStart: Item[], byEnd: Item[]}} orderedItems   All items ordered by start date and by end date
- * @param {Item[]} visibleItems                             The previously visible items.
- * @param {{start: number, end: number}} range              Visible range
- * @return {Item[]} visibleItems                            The new visible items.
- * @private
- */
-ItemSet.prototype._updateVisibleItems = function _updateVisibleItems(orderedItems, visibleItems, range) {
-  var initialPosByStart,
-      newVisibleItems = [],
-      i;
-
-  // first check if the items that were in view previously are still in view.
-  // this handles the case for the ItemRange that is both before and after the current one.
-  if (visibleItems.length > 0) {
-    for (i = 0; i < visibleItems.length; i++) {
-      this._checkIfVisible(visibleItems[i], newVisibleItems);
-    }
-  }
-
-  // If there were no visible items previously, use binarySearch to find a visible ItemPoint or ItemRange (based on startTime)
-  if (newVisibleItems.length == 0) {
-    initialPosByStart = this._binarySearch(orderedItems, range, false);
-  }
-  else {
-    initialPosByStart = orderedItems.byStart.indexOf(newVisibleItems[0]);
-  }
-
-  // use visible search to find a visible ItemRange (only based on endTime)
-  var initialPosByEnd = this._binarySearch(orderedItems, range, true);
-
-  // if we found a initial ID to use, trace it up and down until we meet an invisible item.
-  if (initialPosByStart != -1) {
-    for (i = initialPosByStart; i >= 0; i--) {
-      if (this._checkIfInvisible(orderedItems.byStart[i], newVisibleItems)) {break;}
-    }
-    for (i = initialPosByStart + 1; i < orderedItems.byStart.length; i++) {
-      if (this._checkIfInvisible(orderedItems.byStart[i], newVisibleItems)) {break;}
-    }
-  }
-
-  // if we found a initial ID to use, trace it up and down until we meet an invisible item.
-  if (initialPosByEnd != -1) {
-    for (i = initialPosByEnd; i >= 0; i--) {
-      if (this._checkIfInvisible(orderedItems.byEnd[i], newVisibleItems)) {break;}
-    }
-    for (i = initialPosByEnd + 1; i < orderedItems.byEnd.length; i++) {
-      if (this._checkIfInvisible(orderedItems.byEnd[i], newVisibleItems)) {break;}
-    }
-  }
-
-  return newVisibleItems;
-};
-
 /**
  * Create or delete the group holding all ungrouped items. This group is used when
  * there are no groups specified.
@@ -751,13 +558,14 @@ ItemSet.prototype._onUpdate = function _onUpdate(ids) {
         throw new TypeError('Unknown item type "' + type + '"');
       }
     }
+  });
 
-    if (type == 'range' && me.visibleItems.indexOf(item) == -1) {
-      me._checkIfVisible(item, me.visibleItems);
-    }
+  // reorder the items in all groups
+  // TODO: optimization: only reorder groups affected by the changed items
+  util.forEach(this.groups, function (group) {
+    group._order();
   });
 
-  this._order();
   this.stackDirty = true; // force re-stacking of all items next repaint
   this.emit('change');
 };
@@ -923,12 +731,8 @@ ItemSet.prototype._removeItem = function _removeItem(item) {
   // remove from items
   delete this.items[item.id];
 
-  // remove from visible items
-  var index = this.visibleItems.indexOf(item);
-  if (index != -1) this.visibleItems.splice(index, 1);
-
   // remove from selection
-  index = this.selection.indexOf(item.id);
+  var index = this.selection.indexOf(item.id);
   if (index != -1) this.selection.splice(index, 1);
 
   // remove from group
@@ -937,22 +741,6 @@ ItemSet.prototype._removeItem = function _removeItem(item) {
   if (group) group.remove(item);
 };
 
-/**
- * Order the items
- * @private
- */
-ItemSet.prototype._order = function _order() {
-  var array = util.toArray(this.items);
-  this.orderedItems.byStart = array;
-  this.orderedItems.byEnd = this._constructByEndArray(array);
-
-  //this.orderedItems.byEnd = [].concat(array); // this copies the array
-
-  // reorder the items
-  this.stack.orderByStart(this.orderedItems.byStart);
-  this.stack.orderByEnd(this.orderedItems.byEnd);
-};
-
 /**
  * Create an array containing all items being a range (having an end date)
  * @param array
diff --git a/src/timeline/component/css/groupset.css b/src/timeline/component/css/groupset.css
index 69fb5a52..cbc501b9 100644
--- a/src/timeline/component/css/groupset.css
+++ b/src/timeline/component/css/groupset.css
@@ -1,6 +1,3 @@
-.vis.timeline .groupset {
-  position: relative;
-}
 
 .vis.timeline .labelset {
   position: relative;
@@ -24,15 +21,13 @@
 }
 
 .vis.timeline.bottom .labelset .vlabel,
-.vis.timeline.top .vpanel.side-content,
-.vis.timeline.top .groupset .itemset {
+.vis.timeline.top .vpanel.side-content {
   border-top: 1px solid #bfbfbf;
   border-bottom: none;
 }
 
 .vis.timeline.top .labelset .vlabel,
-.vis.timeline.bottom .vpanel.side-content,
-.vis.timeline.bottom .groupset .itemset {
+.vis.timeline.bottom .vpanel.side-content {
   border-top: none;
   border-bottom: 1px solid #bfbfbf;
 }
diff --git a/src/timeline/component/css/itemset.css b/src/timeline/component/css/itemset.css
index f185baa4..7b75428d 100644
--- a/src/timeline/component/css/itemset.css
+++ b/src/timeline/component/css/itemset.css
@@ -22,3 +22,17 @@
 .vis.timeline .axis {
   overflow: visible;
 }
+
+.vis.timeline  .group {
+  position: relative;
+}
+
+.vis.timeline.top .group {
+  border-top: 1px solid #bfbfbf;
+  border-bottom: none;
+}
+
+.vis.timeline.bottom .group {
+  border-top: none;
+  border-bottom: 1px solid #bfbfbf;
+}
diff --git a/src/timeline/component/item/ItemBox.js b/src/timeline/component/item/ItemBox.js
index dd43d4a8..463814ff 100644
--- a/src/timeline/component/item/ItemBox.js
+++ b/src/timeline/component/item/ItemBox.js
@@ -215,13 +215,13 @@ ItemBox.prototype.repositionY = function repositionY () {
 
     line.style.top = '0';
     line.style.bottom = '';
-    line.style.height = (this.top + 1) + 'px';
+    line.style.height = (this.parent.top + this.top + 1) + 'px';
   }
   else { // orientation 'bottom'
     box.style.top = '';
     box.style.bottom = (this.top || 0) + 'px';
 
-    line.style.top = (this.parent.getBackgroundHeight() - this.top - 1) + 'px';
+    line.style.top = (this.parent.top + this.parent.height - this.top - 1) + 'px';
     line.style.bottom = '0';
     line.style.height = '';
   }
diff --git a/src/timeline/stack.js b/src/timeline/stack.js
new file mode 100644
index 00000000..06050f43
--- /dev/null
+++ b/src/timeline/stack.js
@@ -0,0 +1,96 @@
+/**
+ * Utility functions for ordering and stacking of items
+ */
+var stack = {};
+
+/**
+ * Order items by their start data
+ * @param {Item[]} items
+ */
+stack.orderByStart = function orderByStart(items) {
+  items.sort(function (a, b) {
+    return a.data.start - b.data.start;
+  });
+};
+
+/**
+ * Order items by their end date. If they have no end date, their start date
+ * is used.
+ * @param {Item[]} items
+ */
+stack.orderByEnd = function orderByEnd(items) {
+  items.sort(function (a, b) {
+    var aTime = ('end' in a.data) ? a.data.end : a.data.start,
+        bTime = ('end' in b.data) ? b.data.end : b.data.start;
+
+    return aTime - bTime;
+  });
+};
+
+/**
+ * Adjust vertical positions of the events such that they don't overlap each
+ * other.
+ * @param {Item[]} items
+ *            All visible items
+ * @param {{item: number, axis: number}} margin
+ *            Margins between items and between items and the axis.
+ * @param {boolean} [force=false]
+ *            If true, all items will be re-stacked. If false (default), only
+ *            items having a top===null will be re-stacked
+ */
+stack.stack = function _stack (items, margin, force) {
+  var i, iMax;
+
+  if (force) {
+    // reset top position of all items
+    for (i = 0, iMax = items.length; i < iMax; i++) {
+      items[i].top = null;
+    }
+  }
+
+  // calculate new, non-overlapping positions
+  for (i = 0, iMax = items.length; i < iMax; i++) {
+    var item = items[i];
+    if (item.top === null) {
+      // initialize top position
+      item.top = margin.axis;
+
+      do {
+        // TODO: optimize checking for overlap. when there is a gap without items,
+        //       you only need to check for items from the next item on, not from zero
+        var collidingItem = null;
+        for (var j = 0, jj = items.length; j < jj; j++) {
+          var other = items[j];
+          if (other.top !== null && other !== item && stack.collision(item, other, margin.item)) {
+            collidingItem = other;
+            break;
+          }
+        }
+
+        if (collidingItem != null) {
+          // There is a collision. Reposition the event above the colliding element
+          item.top = collidingItem.top + collidingItem.height + margin.item;
+        }
+      } while (collidingItem);
+    }
+  }
+};
+
+/**
+ * Test if the two provided items collide
+ * The items must have parameters left, width, top, and height.
+ * @param {Item} a          The first item
+ * @param {Item} b          The second item
+ * @param {Number} margin   A minimum required margin.
+ *                          If margin is provided, the two items will be
+ *                          marked colliding when they overlap or
+ *                          when the margin between the two is smaller than
+ *                          the requested margin.
+ * @return {boolean}        true if a and b collide, else false
+ */
+stack.collision = function collision (a, b, margin) {
+  return ((a.left - margin) < (b.left + b.width) &&
+      (a.left + a.width + margin) > b.left &&
+      (a.top - margin) < (b.top + b.height) &&
+      (a.top + a.height + margin) > b.top);
+};