title: Sorting items with groupOrder visjs timeline
body: <p>The visjs timeline groupOrder example appears to be able to sort items, but I don't see the code that actually accomplishes this – I only see the code that actually sorts the groups (if it is possible to sort the items with groupOrder that seems preferable to 'item ordering' because the latter has a performance limitation of 300ish items). What is responsible for ordering 'item 2', 'item 3' and 'item 4' in this example?  <a href="http://jsfiddle.net/5p0ggwkx/1369/" rel="nofollow noreferrer">http://jsfiddle.net/5p0ggwkx/1369/</a></p>
  
  <pre><code>&lt;div id="visualization"&gt;&lt;/div&gt;
  
    var groups = new vis.DataSet([
      {id: 0, content: 'First', value: 1},
      {id: 1, content: 'Third', value: 3},
      {id: 2, content: 'Second', value: 2}
    ]);
  
    // create a dataset with items
    // note that months are zero-based in the JavaScript Date object, so month 3 is April
    var items = new vis.DataSet([
    {id: 1, group: 0, content: 'item 1', start: new Date(2014, 3, 19), end: new Date(2014, 3, 20)},
  
      {id: 0, group: 0, content: 'item 0', start: new Date(2014, 3, 17), end: new Date(2014, 3, 21)},
  {id: 2, group: 1, content: 'item 2', start: new Date(2014, 3, 16), end: new Date(2014, 3, 24)},
   {id: 3, group: 1, content: 'item 3', start: new Date(2014, 3, 16), end: new Date(2014, 3, 24)},
  {id: 4, group: 1, content: 'item 4', start: new Date(2014, 3, 16), end: new Date(2014, 3, 26)},
  
  {id: 5, group: 2, content: 'item 5', start: new Date(2014, 3, 24), end: new Date(2014, 3, 27)}
    ]);
  
    // create visualization
    var container = document.getElementById('visualization');
    var options = {
      // option groupOrder can be a property name or a sort function
      // the sort function must compare two groups and return a value
      //     &gt; 0 when a &gt; b
      //     &lt; 0 when a &lt; b
      //       0 when a == b
      groupOrder: function (a, b) {
        return a.value - b.value;
      },
      editable: true
    };
  
    var timeline = new vis.Timeline(container);
    timeline.setOptions(options);
    timeline.setGroups(groups);
    timeline.setItems(items);
  </code></pre>