title: Cannot read property "getPropertyValue" of null in xChart
body: <p>I am trying to use d3-based charting library xChart to create a graph module in my webapp. I am just using the example data provided by the xChart documentation, and I get this error every time I try to create the chart: </p>
  
  <pre><code>Uncaught TypeError: Cannot read property 'getPropertyValue' of null 
  </code></pre>
  
  <p>The error is on line 666 (lol) of d3.js in the getComputedStyle method. </p>
  
  <pre><code>if (n &lt; 2) return d3_window.getComputedStyle(this.node(), null).getPropertyValue(name);
  </code></pre>
  
  <p>My JavaScript is as follows: </p>
  
  <pre><code>var data = {
      xScale: "ordinal",
      yScale: "linear",
      main: [
        {
          className: ".pizza",
          data: [
            {
              x: "Pepperoni",
              y: 4
            }, {
              x: "Cheese",
              y: 8
            }
          ]
        }, {
          className: ".pizza",
          data: [
            {
              x: "Pepperoni",
              y: 6
            }, {
              x: "Cheese",
              y: 5
            }
          ]
        }
      ]
    };
    var myChart = new xChart('bar', data, '#graph-figure');
  </code></pre>
  
  <p>And my HTML is</p>
  
  <pre><code>&lt;figure id="graph-figure"&gt;&lt;/figure&gt;
  </code></pre>
  
  <p>Can anyone help me out with this?</p>