title: Cumulative Sum by date (Month)
body: <p>I have a pandas dataframe and I need to work out the cumulative sum for each month. </p>
  
  <pre><code>Date    Amount
  2017/01/12  50
  2017/01/12  30
  2017/01/15  70
  2017/01/23  80
  2017/02/01  90
  2017/02/01  10
  2017/02/02  10
  2017/02/03  10
  2017/02/03  20
  2017/02/04  60
  2017/02/04  90
  2017/02/04  100
  </code></pre>
  
  <p>The cumulative sum is the trailing sum for each day i.e 01-31. However, some days are missing. The data frame should look like </p>
  
  <pre><code>Date    Sum_Amount
  2017/01/12  80
  2017/01/15  150
  2017/01/23  203
  2017/02/01  100
  2017/02/02  110
  2017/02/03  140
  2017/02/04  390
  </code></pre>