title: How can I work around hovertool displaying entire column with multi_line?
body: <p>Bokeh plots made with multi_line have an issue with hovertools where the tooltips display the entire column of values instead of the values at a specific coordinate. This is a known issue on the Github repo: <a href="https://github.com/bokeh/bokeh/issues/7969" rel="nofollow noreferrer">https://github.com/bokeh/bokeh/issues/7969</a></p>
  <p>It's possible to work around this for x- and y-values by using the special fields <code>$data_x</code> and <code>$data_y</code> to get the values. However, I have not found a way to work around this issue to display values from a different column in the ColumnDataSource used to plot the multi_line. Here's a minimal example to show what I mean:</p>
  <pre><code>from bokeh.plotting import figure
  from bokeh.io import show
  from bokeh.models import ColumnDataSource, HoverTool
  
  x = [[1,2,3,4], [1,2,3,4]]
  y = [[2,3,4,5], [3,2,0,-1]]
  additional_col = [['a', 'b', 'c', 'd'], ['a', 'b', 'c', 'd']]
  
  source = ColumnDataSource({&quot;xs&quot;: x, &quot;ys&quot;: y, &quot;additional_col&quot;: additional_col})
  hover = HoverTool(tooltips=[(&quot;x&quot;, &quot;$data_x&quot;),
                              (&quot;y&quot;, &quot;$data_y&quot;),
                              (&quot;Additional data&quot;, &quot;@additional_col&quot;)])
  
  fig = figure(tools=[hover])
  fig.multi_line(&quot;xs&quot;, &quot;ys&quot;, source=source)
  
  show(fig)
  </code></pre>
  <p>And the result:</p>
  <p><a href="https://i.stack.imgur.com/opzbf.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/opzbf.png" alt="Plot result" /></a></p>
  <p>As you can see, the x- and y-values are displayed just fine with the workaround, but for the additional column the entire column is displayed. Is there a workaround available to display the value associated with the specific coordinate?</p>