title: Count the total of cases
body: <p>I had a dataframe as below by recording the number of cases per month:</p>
  
  <pre><code>Month         client      Cases       
  2019-06-01      A           1
  2019-05-01      A           0
  2019-04-01      A           0
  2019-03-01      A           2
  2019-02-01      A           0
  2019-06-01      B           1
  2019-05-01      B           1
  2019-04-01      B           0
  2019-03-01      B           0
  2019-02-01      B           0
  </code></pre>
  
  <p>I would like to accumulated number per month by clients as below:</p>
  
  <pre><code>Month         client      Cases       
  2019-06-01      A           3
  2019-05-01      A           2
  2019-04-01      A           2
  2019-03-01      A           2
  2019-02-01      A           0
  2019-06-01      B           2
  2019-05-01      B           1
  2019-04-01      B           0
  2019-03-01      B           0
  2019-02-01      B           0
  </code></pre>