title: multi-monthly mean with pandas' Series
body: <p>I have a sequence of <code>datetime</code> objects and a series of data which spans through several years. A can create a <code>Series</code> object and resample it to group it by months:</p>
  
  <pre><code>df=pd.Series(varv,index=dates)
  multiMmean=df.resample("M", how='mean')
  print multiMmean
  </code></pre>
  
  <p>This, however, outputs </p>
  
  <pre><code>2005-10-31    172.4
  2005-11-30     69.3
  2005-12-31    187.6
  2006-01-31    126.4
  2006-02-28    187.0
  2006-03-31    108.3
  ...
  2014-01-31     94.6
  2014-02-28     82.3
  2014-03-31    130.1
  2014-04-30     59.2
  2014-05-31     55.6
  2014-06-30      1.2
  </code></pre>
  
  <p>which is a list of the mean value for each month of the series. This is not what I want. I want 12 values, one for every month of the year with a mean for each month through the years. How do I get that for <code>multiMmean</code>?</p>
  
  <p>I have tried using <code>resample("M",how='mean')</code> on <code>multiMmean</code> and list comprehensions but I cannot get it to work. What am I missing?</p>
  
  <p>Thank you.</p>