title: How to prevent vis.js timeline "customTime" item move?
body: <p>I coded timeline via vis.js.
  And I appended custom time item using by addCustomTime().</p>
  
  <p>My situation is.</p>
  
  <p>I want add fixed custom time, but It move when I drag it.
  Anbybody know how to prevent custom item move?</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>  // DOM element where the Timeline will be attached
    var container = document.getElementById('visualization');
  
    // Create a DataSet (allows two way data-binding)
    var items = new vis.DataSet([
      {id: 1, content: 'item 1', start: '2013-04-20'},
    ]);
  
    // Configuration for the Timeline
    var options = {};
  
    // Create a Timeline
    var timeline = new vis.Timeline(container, items, options);
    
    timeline.addCustomTime('2013-04-21'); // I want this item not moving.</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;link href="https://cdnjs.cloudflare.com/ajax/libs/vis/4.19.1/vis.min.css" rel="stylesheet"/&gt;
  &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/vis/4.19.1/vis-timeline-graph2d.min.js"&gt;&lt;/script&gt;
  
  &lt;div id="visualization"&gt;&lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>