title: Bokeh vbar with group counts after filtering with a CDSView
body: <p>I'd like to create a Bokeh vertical bar chart (<code>vbar</code>) coupled with a scatter plot. I need to start from a given <code>ColumnDataSource</code> (then filtered with a <code>CDSView</code>), as this is only a part of a complex visualization and I need all the plots to be linked together.</p>
  <p>The only tricky part for which I didn't find anything about after extensive research is: <strong>how to show counts <strong>(i.e., the values provided in the <code>top</code> parameter of <code>vbar</code>)</strong> based only on data points that are both filtered AND selected in the scatter plot</strong>? Both the plots receive the same filters from which a view is created.</p>
  <p>The documentation shows how to link selected points in different plots (by using the same <code>ColumnDataSource</code> object), how to filter them, and how to use <code>vbar</code>, but not all of them together.</p>
  <p>Any hint? Thank you very much in advance.</p>