title: How to make two sets of axis labels in grouped line graph in dimple.js?
body: <p>In the example here, </p>
  
  <p><a href="http://dimplejs.org/examples_viewer.html?id=lines_horizontal_grouped_stacked" rel="nofollow">http://dimplejs.org/examples_viewer.html?id=lines_horizontal_grouped_stacked</a></p>
  
  <p>How can I add x-labels of month, like just above "Black Mesa" and "Aperture"?</p>