title: Plot a stacked bar graph from two pandas groupby objects?
body: <p>Consider two input files <code>b.dat</code> and <code>c.dat</code>:</p>
  
  <pre><code>string,date,number
  a string,2/5/11 9:16am,1.0
  a string,3/5/11 10:44pm,2.0
  a string,4/22/11 12:07pm,3.0
  a string,4/22/11 12:10pm,4.0
  a string,4/29/11 11:59am,1.0
  a string,5/2/11 1:41pm,2.0
  a string,5/2/11 2:02pm,3.0
  a string,5/2/11 2:56pm,4.0
  a string,5/2/11 3:00pm,5.0
  a string,5/2/14 3:02pm,6.0
  a string,5/2/14 3:18pm,7.0
  
  
  string,date,number
  a string,2/5/10 9:16am,4.0
  a string,3/4/10 10:44pm,5.0
  a string,4/22/10 12:07pm,3.0
  a string,6/22/10 12:10pm,6.0
  a string,4/29/11 11:59am,1.0
  a string,5/2/11 1:41pm,9.0
  a string,5/27/11 2:02pm,3.0
  a string,6/2/11 2:56pm,14.0
  a string,5/2/11 3:00pm,5.0
  a string,5/8/14 3:02pm,16.0
  a string,5/2/14 3:18pm,7.0
  </code></pre>
  
  <p>I can import these and group into monthly totals:</p>
  
  <pre><code>b=pd.read_csv('b.dat')
  c=pd.read_csv('c.dat')
  b.index=b['date']
  c.index=c['date']
  b['date']=pd.to_datetime(b['date'],format='%m/%d/%y %I:%M%p')
  c['date']=pd.to_datetime(c['date'],format='%m/%d/%y %I:%M%p')
  bg=pd.groupby(b,by=[b.index.year,b.index.month])
  cg=pd.groupby(c,by=[c.index.year,c.index.month])
  </code></pre>
  
  <p>Next I'd like to plot a stacked bar graph. However my attempts result in separate graphs.</p>
  
  <pre><code>bg.sum().plot(kind='bar',stacked=True)
  cg.sum().plot(kind='bar',stacked=True)
  </code></pre>
  
  <p>Does anyone know how this can be done?</p>