title: Sort bars on nvd3 charts
body: <p>Is there a easy way to sort <em>bars</em> (ascending or descending) on <strong>Nvd3</strong> <em>discreteBarChart</em> or on the <em>HorizontalBarChart</em>.
  I think a function should be added to the main lib (Nvd3) so that the option can be used as 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>.sortAscending(true) //Used for ascending bars according to their value
    or
  .sortDescending(true)  //Used for descending bars according to their value</code></pre>
  </div>
  </div>
  </p>