title: Can't read json in D3
body: <p>I am trying to read json file using D3 library but when i try to read json i get alert Null Values (here i am using alert).</p>
  
  <pre><code>&lt;!doctype html&gt;
  &lt;html lang="en"&gt;
  &lt;head&gt;
      &lt;meta charset="utf-8"&gt;
      &lt;title&gt;Simple venn.js example&lt;/title&gt;
  &lt;/head&gt;
  
  &lt;body&gt;
      &lt;div id="weighted_example"&gt;&lt;/div&gt;
  &lt;/body&gt;
  
  &lt;script type="text/javascript" src="https://code.jquery.com/jquery-1.9.1.js"&gt;&lt;/script&gt;
  &lt;script src="https://d3js.org/d3.v4.min.js"&gt;&lt;/script&gt;
  
  &lt;script&gt;
  d3.json( "file.json", function( json ) {
    alert( "JSON Data: " + json );
  });
  &lt;/script&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>Following is file.json (which is already validated online)</p>
  
  <pre><code> [{
          "sets": [0],
          "size": 1958
      },
      {
          "sets": [1],
          "size": 1856
      },
      {
          "sets": [2],
          "size": 1297
      },
      {
          "sets": [0, 1],
          "size": 220
      },
      {
          "sets": [2, 0],
          "size": 123
      },
      {
          "sets": [2, 1],
          "size": 139
      }
   ]
  </code></pre>
  
  <p>How can i fix such issue?</p>