Updated history and docs

HISTORY.md

@@ -19,6 +19,9 @@ http://visjs.org
 
 - Implemented option `itemsAlwaysDraggable`, See #1395. Thanks @liuqingc.
 - Implemented option `multiselectPerGroup`. Thanks @hansmaulwurf23.
+- Implemented property `oldData` on change events of the DataSet, and
+  deprecated the `data` property which wrongly contained new data instead of
+  old data. Thanks @hansmaulwurf23.
 - Expose `vis.timeline.Core` for customization purposes.
 
 ### Graph2d

docs/data/dataset.html

@@ -598,9 +598,10 @@ function (event, properties, senderId) {
         <code>update</code>, and <code>remove</code>,
         <code>properties</code> is always an object containing a property
         <code>items</code>, which contains an array with the ids of the affected
-        items. The <code>update</code> event has an extra field <code>data</code>
-        containing the original data of the updated items, i.e. the gives the
-        changed fields of the changed items.
+        items. The <code>update</code> event has an extra field <code>oldData</code>
+        containing the original data of the updated items, which allows to
+        determine the changed fields of the changed items when comparing to
+        the new item properties.
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