title: Multiple lines between nodes and hierarchical layout in vis.js
body: <p>I have a vis.js network graph that contains multiple edges between nodes and I am trying to have it setup with a hierarchical layout as well. If I do a normal graph without the hierarchical layout options then multiple lines are shown between nodes, however, once I turn on/place in the hierarchical layout option only one line is drawn.</p>
  
  <p>Below are two plunkers showing what I mean:</p>
  
  <p><a href="https://plnkr.co/edit/c8SuBc0XjDZnn6im2vdg" rel="noreferrer">https://plnkr.co/edit/c8SuBc0XjDZnn6im2vdg</a> (Plunker with hierarchical layout off)</p>
  
  <pre><code>var options = { 
      height: '400px', 
      edges: { smooth: true, shadow: true},
      layout: {
         randomSeed: 1,
         improvedLayout: true,
         hierarchical: {
             enabled: false, //change to true to see the other graph
             direction: 'UD',
             sortMethod: 'directed'
        }
     }
  }
  </code></pre>
  
  <p><a href="https://plnkr.co/edit/L22bHk6kh1XKXulTjESD" rel="noreferrer">https://plnkr.co/edit/L22bHk6kh1XKXulTjESD</a> (Plunker with hierarchical layout on)</p>
  
  <pre><code>var options = { 
      height: '400px', 
      edges: { smooth: true, shadow: true},
      layout: {
         randomSeed: 1,
         improvedLayout: true,
         hierarchical: {
             enabled: true, //change to true to see the other graph
             direction: 'UD',
             sortMethod: 'directed'
        }
     }
  }
  </code></pre>
  
  <p>As you can see, in the second one 2 edges from node 1 to node 3 collapse into one visually. Any help would be great.</p>