Fixed a reference to DataSet

lib/timeline/Graph2d.js

@@ -20,7 +20,7 @@ var LineGraph = require('./component/LineGraph');
  */
 function Graph2d (container, items, groups, options) {
   // if the third element is options, the forth is groups (optionally);
-  if (!(groups instanceof Array || groups instanceof vis.DataSet) && groups instanceof Object) {
+  if (!(groups instanceof Array || groups instanceof DataSet) && groups instanceof Object) {
     var forthArgument = options;
     options = groups;
     groups = forthArgument;

lib/timeline/Timeline.js

@@ -24,7 +24,7 @@ function Timeline (container, items, groups, options) {
   }
 
   // if the third element is options, the forth is groups (optionally);
-  if (!(groups instanceof Array || groups instanceof vis.DataSet) && groups instanceof Object) {
+  if (!(groups instanceof Array || groups instanceof DataSet) && groups instanceof Object) {
     var forthArgument = options;
     options = groups;
     groups = forthArgument;