title: Bokeh or Holoviews: BarChart by downsampling date time to months/years/etc
body: <p>I have a pandas dataframe (wrapped in holoviews typically) that has three columns.  Col1 is a datetime, Col2 are categorical strings (i.e. one of 'Cat', 'Dog', 'Bird'), and Col3 being an integer count.  </p>
  
  <p>I'm trying to find a way to use the bokeh library to downsample the datetime to months, quarters, years, etc. similar to that available in pandas.DataFrame.resample or pandas.DataFrame.groupby([pd.Grouper(key='Date', freq=sample)]).  </p>
  
  <p>Does anyone know if there is a native bokeh ability to do this, or do I need to provide all the data already sampled from pandas?  Thanks!</p>