fixed bug with empty groups in graph2d

lib/timeline/component/LineGraph.js

@@ -602,9 +602,15 @@ LineGraph.prototype._updateGraph = function () {
             }
           }
           // preprocess, split into ranges and data
-          preprocessedGroup = this._preprocessData(groupData, group);
-          groupRanges.push({min: preprocessedGroup.min, max: preprocessedGroup.max});
-          preprocessedGroupData.push(preprocessedGroup.data);
+          if (groupData.length > 0) {
+            preprocessedGroup = this._preprocessData(groupData, group);
+            groupRanges.push({min: preprocessedGroup.min, max: preprocessedGroup.max});
+            preprocessedGroupData.push(preprocessedGroup.data);
+          }
+          else {
+            groupRanges.push({});
+            preprocessedGroupData.push([]);
+          }
         }
         else {
           groupRanges.push({});