title: Missing links in force directed graph
body: <p>I'm using <a href="https://bl.ocks.org/mbostock/0adcc447925ffae87975a3a81628a196" rel="nofollow noreferrer">Modifying a Force Directed Graph II</a> as a base for my graph. I thought I'd coped with the different ways we're handling how links are formatted (Bostock's uses references, mine uses strings), by changing</p>
  
  <pre><code>simulation.force("link", d3.forceLink(links).distance(200))
  </code></pre>
  
  <p>to</p>
  
  <pre><code>simulation.force("link", d3.forceLink().id(function(d) {
      return d.id;
  }))
  </code></pre>
  
  <p>but no dice. The links still aren't being drawn. What else am I missing?</p>
  
  <p><a href="https://jsfiddle.net/qd1dsque/3/" rel="nofollow noreferrer">My jsfiddle is here.</a></p>