title: How to display difference between two points from two different glyphs in Bokeh using HoverTool?
body: <p>I have two lines in a plot and I want to displace the difference of each pair of points on hover using <code>HoverTool</code>.</p>
  
  <p>I tried <code>'@y' - '@yhat'</code>, <code>float('@y') - float('@yhat')</code>, <code>@y - @yhat</code>, but nothing worked.</p>