title: grouping by column and then doing a boxplot by the index in pandas
body: <p>I have a large dataframe which I would like to group by some column and examine graphically the distribution per group using a boxplot. I found that <code>df.boxplot()</code> will do it for each column of the dataframe and put it in one plot, just as I need. </p>
  
  <p>The problem is that after a groupby operation, my data is all in one column with the group labels in the index , so i can't call boxplot on the result. </p>
  
  <p>here is an example: </p>
  
  <pre><code>df = DataFrame({'a':rand(10),'b':[x%2 for x in range(10)]})
  df
  
           a   b
  0    0.273548    0
  1    0.378765    1
  2    0.190848    0
  3    0.646606    1
  4    0.562591    0
  5    0.409250    1
  6    0.637074    0
  7    0.946864    1
  8    0.203656    0
  9    0.276929    1
  </code></pre>
  
  <p>Now I want to group by column b and boxplot the distribution of both groups in one boxplot. How can I do that?</p>