title: JSON call error with d3js
body: <p>I'm trying to replicate <a href="http://plnkr.co/edit/rtaW4mBcUN9bepq8p4Bo?p=preview" rel="nofollow">this example</a> of a bar chart that transitions in d3js. However, when I input my data into the .json file I get the error "Cannot call method 'forEach' of undefined" so I'm pretty sure I'm calling the .json file incorrectly. I have correctly changed the names of my variables, which are dates instead of text, so maybe that's messing something up: </p>
  
  <p>Here's my .json file:</p>
  
  <pre><code>[{"network":"ABC","2002":9860,"2003":10596,"2004":9989,"2005":12217,"2006":12281,"2007":11913,"2008":12265,"2009":11050,"2010":9595,"2011":9871,"2012":8339},
  {"network":"AZA","2002":0,"2003":0,"2004":0,"2005":0,"2006":213,"2007":0,"2008":0,"2009":201,"2010":236,"2011":212,"2012":0},
  {"network":"CBS","2002":13959,"2003":13856,"2004":13581,"2005":12843,"2006":13019,"2007":11726,"2008":11274,"2009":11812,"2010":12538,"2011":12284,"2012":10690}]
  </code></pre>
  
  <p>And how I've tried to call it:</p>
  
  <pre><code>var json 
  
  d3.json("data.json", function(error, result){
    json = result;
    var data = [];
          json.forEach(function(d){
              data.push({variable: +d['2002'], network: d['network']});
           });
    x.domain(data.map(function(d) { return d.network; }));
    y.domain([d3.min(data, function(d) { return d.variable; }), d3.max(data, function(d) {      return d.variable; })]);
  </code></pre>
  
  <p>Any help would be much appreciated! </p>