title: cumsum() on multi-index pandas dataframe
body: <p>I have a multi-index dataframe that shows the sum of transactions on a monthly frequency. </p>
  
  <p>I am trying to get a cumsum() on yearly basis that respects my 'mapid' and 'service' multi-index. However I don't know how to derive that data</p>
  
  <pre><code>combined_df = combined_df.groupby([pd.Grouper(freq='M'), 'provider', 'mapid', 'service']).sum()
  
  
                                                   cost
  datetime   provider mapid   service                  
  2017-08-31 Amazon   10147.0 Monitor              0.41
                              Storage             90.51
                              Virtual Machine  11646.32
  2017-09-30 Amazon   10147.0 Monitor              0.89
                              Storage            226.06
                              Virtual Machine  32624.91
  2017-10-31 Amazon   10147.0 Monitor              0.17
                              Storage            261.72
                              Virtual Machine  36934.93
  2017-11-30 Amazon   10147.0 Monitor              0.35
                              Storage            269.06
                              Virtual Machine  30790.70
  </code></pre>
  
  <p>I would like to derive the follow results </p>
  
  <p><a href="https://i.stack.imgur.com/KxoWy.png" rel="noreferrer"><img src="https://i.stack.imgur.com/KxoWy.png" alt="enter image description here"></a></p>