title: Boxplot with groupby two features
body: <p>Say I have a df consisting of 3 columns ['Y/N','Test Before','Test After'], where 'Y/N' is boolean and 'Test Before,' 'Test After' is float.</p>
  
  <p>eg, </p>
  
  <pre><code>'Y/N'    'Test Before'    'Test After'
  True         75               100
  False        75                50
  True         50                60
  False        50                40
    ...
  </code></pre>
  
  <p>I can use <code>df.boxplot(column='Test Before/Test After', by 'Y/N')</code> to create separate boxplots of 'column name' each grouped by 'Y/N.'  </p>
  
  <p>However,  I would like to create separate boxplots of boolean 'Y/N,' grouped by 'Test Before,' 'Test After,' eg: </p>
  
  <p>Boxplot A contains boxplots of df['Y/N'] == True for x values df['Test Before'] and df['Test After'].   </p>
  
  <p>Boxplot B contains boxplots of df['Y/N'] == False for x values df['Test Before'] and df['Test After'].</p>