title: How do I extract data from a Bokeh ColumnDatasource
body: <p>I was trying to avoid using a ColumnDataSource and instead of that I was passing pandas dataframe columns directly to Bokeh plots.</p>
  
  <p>Soon though I had to implement a HoverTool which requires to have the data in a ColumnDataSource. So, I started using ColumnDataSource.</p>
  
  <p>Now, I was creating a box annotation and I had to use the maximum value of a certain column from my data to define the top border of the box.</p>
  
  <p>I can do that easily using pandas:</p>
  
  <pre><code>low_box = BoxAnnotation(
      top=flowers['petal_width'][flowers['species']=='setosa'].max(),
      fill_alpha=0.1, fill_color='red')
  </code></pre>
  
  <p>But I can't figure out how to extract the maximum from a ColumnDataSource.</p>
  
  <p>Is there a way to extract a maximum value from it, or is my approach all wrong in the first place?</p>