title: vis.js timeline, don't stack items with no time overlap
body: <p>I'm using vis.js to display a timeline.</p>
  
  <p>I have the following items:</p>
  
  <pre><code>var items = new vis.DataSet([
    {id: 1, content: '1) Next To 2', start: '2014-04-20 00:00:00', end : '2014-04-20 00:59:59'},
    {id: 2, content: '2) Next To 1', start: '2014-04-20 01:00:00', end : '2014-04-20 02:59:59'},
    {id: 3, content: 'Underneath   ', start: '2014-04-20 00:00:00', end : '2014-04-20 05:59:59'}
  ]);
  </code></pre>
  
  <p>id 1 and id 2 start/end do not overlap each other (timewise). So I <strong>always</strong> want them to appear on the same line within the timeline regardless of the zoom level.</p>
  
  <p>However I can't set <code>stack : false</code>, as I want <code>id : 3</code> to be underneath both 1 and 2.</p>
  
  <p>Here is a JSFiddle: <a href="http://jsfiddle.net/uqn6q4jd/17/" rel="noreferrer">http://jsfiddle.net/uqn6q4jd/17/</a></p>
  
  <p>1) and 2) should always be on the same line,
  3) always underneath</p>
  
  <p>Can I accomplish this anyway?</p>
  
  <p>I have had a look at the Vis JS source, and feel I could probably achieve what I need via alterations to:</p>
  
  <pre><code>exports.stack = function...
  exports.nostack = function...
  </code></pre>
  
  <p>If there's a setting or feature I am missing that would be the preferred route than me making changes...</p>