title: Can't visualize nodes in d3 force layout
body: <p>I'm trying to change my visualization script to be more like the <a href="http://bl.ocks.org/mbostock/1095795" rel="nofollow">Modifying a Force Layout Example</a>. Since I don't have fixed nodes like <code>a, b and c</code> to add I read a <code>json file</code> to fill the <code>nodes</code> and <code>links</code> array.</p>
  
  <pre><code>d3.json("mock.json", function(error, json) {
      if (error)
          throw error;
      nodes = nodes.concat(json.nodes);
      links = links.concat(json.links);
      start();
  });
  </code></pre>
  
  <p><code>nodes</code> and <code>links</code> have the right size, meaning nodes contains <code>26 nodes</code> and links <code>37 links</code>. Now I want to simply visualize them using <code>line</code> and <code>circle</code> elements.</p>
  
  <pre><code>function start() {
      link = link.data(force.links(), function(d) { return d.source.id + "-" + d.target.id; });
      link.enter().append("line").attr("class", "link");
      link.exit().remove();
  
      node = node.data(force.nodes(), function(d) { return d.id;});
      node.enter().append("circle").attr("class", "node").attr("r", 8);
      node.exit().remove();
      force.start();
  }
  </code></pre>
  
  <p>This is very simular to the example and I don't really understand why this won't work. I provide <a href="https://plnkr.co/edit/63wrSbzJFx88VhiutmOm?p=preview" rel="nofollow">a demo</a> with the mock. Is there an issue because I use <code>concat()</code> and not <code>push()</code> or is there anything else wrong?</p>