to_bokeh should have an optional group_by parameter

[okennedy]

Bokeh naturally interprets Pandas group-by objects, making it very easy to plot series for datasets where each row defines one data point for a single group. As an example, take the Vizier Getting Started Bokeh Example.

Over half the code in the script is used solely to break the dataset down into groups.

By comparison, if the dataset were coming from Pandas, the following line would supplant nearly everything:

source = ds.groupby(['COUNTY'])

I propose adding a group_by optional parameter for to_bokeh so that the following line has the same behavior:

source = vizierdb.get_dataset('ticks').to_bokeh(group_by = ['COUNTY'])