title: Google Bubble Chart custom tooltip column does not render
body: <p>I am trying to add a custom tooltip to a Bubble chart, to replace the default tooltip.  I have followed the instructions from the docs site (<a href="https://developers.google.com/chart/interactive/docs/customizing_tooltip_content" rel="nofollow">here</a>) to add a new <code>string</code> column to the DataTable, with <code>role: 'tooltip'</code>.  However, you can see in the following JS fiddle example, that the custom tooltip content does not render.  The chart still shows the default tooltip.  </p>
  
  <p>Anyone know what I still need to do to get this custom tooltip content to show up?  </p>
  
  <p><a href="http://jsfiddle.net/MPBmY/2/" rel="nofollow">http://jsfiddle.net/MPBmY/2/</a></p>