visjstimeline-当同一分组中的项在同一行时

下面是一个例子，展示了如何在同一行上显示同一分组中的项：

var container = document.getElementById('visualization');

var items = new vis.DataSet([
  {id: 1, group: 1, content: 'Item 1', start: '2014-04-20'},
  {id: 2, group: 2, content: 'Item 2', start: '2014-04-14'},
  {id: 3, group: 1, content: 'Item 3', start: '2014-04-18'},
  {id: 4, group: 3, content: 'Item 4', start: '2014-04-16', end: '2014-04-19'},
  {id: 5, group: 2, content: 'Item 5', start: '2014-04-25'},
  {id: 6, group: 4, content: 'Item 6', start: '2014-04-27'}
]);

var groups = new vis.DataSet([
  {id: 1, content: 'Group 1'},
  {id: 2, content: 'Group 2'},
  {id: 3, content: 'Group 3'},
  {id: 4, content: 'Group 4'}
]);

var options = { 
  groupOrder: 'id',
  groupOrderSwap: function (a, b, groups) {
    var aGroup = groups.get(a);
    var bGroup = groups.get(b);

    // if both items are in the same group, use the item with the smallest sum
    if (aGroup && bGroup && aGroup.id == bGroup.id) {
      var aStart = new Date(items.get(a).start).getTime();
      var bStart = new Date(items.get(b).start).getTime();
      var aEnd   = new Date(items.get(a).end).getTime();
      var bEnd   = new Date(items.get(b).end).getTime();
      var aDiff  = (aEnd) ? (aEnd - aStart) : 0;
      var bDiff  = (bEnd) ? (