title: What is the source of the index in a ColumnDataSource created from a pandas DataFrame?
body: <p>Dataframes naturally come with an index, i.e. those set of row headers we can think of them as. When I construct a ColumnDataSource in bokeh to capture the information in that dataframe for plotting and annotating in a HoverTool I see that the hover tool has a built in <code>( "index" , "$index" )</code> tooltip available. Will this index be identical to my dataframe's index or is it simply the row index in the ColumnDataSource</p>