title: How to sort x axis in desc order
body: <p>Is it possible to sort x-axis in desc order.  Naturally c3 is sorting the x axis asc order.  My graph is like below.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>jQuery(function($) {
    var chart = c3.generate({
      bindto: '#chart',
      data: {
        x: 'x',
        columns: [
          ['x', 20, 19, 18, 17, 16, 15, 14, 13, 12, 11],
          ['data1', 180, 232, 250, 289, 424, 473, 414, 428, 590, 600]
        ],
        axes: {
          data1: 'y2'
        },
        regions: {
          'data1': [{start: 16, style: 'dashed'}]
        }
      },
      axis: {
        y: {
          show: false
        },
        y2: {
          show: true
        }
      }
    });
  });</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;link href="http://cdnjs.cloudflare.com/ajax/libs/c3/0.3.0/c3.min.css" rel="stylesheet"/&gt;
  
  &lt;div id="chart"&gt;&lt;/div&gt;
  
  
  &lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"&gt;&lt;/script&gt;
  &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js"&gt;&lt;/script&gt;
  &lt;script src="http://cdnjs.cloudflare.com/ajax/libs/c3/0.3.0/c3.min.js"&gt;&lt;/script&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>I want the graph x axis to be exactly as my x series like in descending order.  Any help would be greatly appreciated.</p>