title: pandas sort column values
body: <p>ie)</p>
  
  <pre><code>        count
  2015-01  2
  2015-02  1
  2015-03  4
  </code></pre>
  
  <p>for the group i tried pd.groupby(b,by=[b.index.month,b.index.year])</p>
  
  <p>but there was object has no attribute 'month' error</p>