title: Pandas groupby cumulative sum
body: <p>I would like to add a cumulative sum column to my Pandas dataframe so that:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>name</th>
  <th>day</th>
  <th>no</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>Jack</td>
  <td>Monday</td>
  <td>10</td>
  </tr>
  <tr>
  <td>Jack</td>
  <td>Tuesday</td>
  <td>20</td>
  </tr>
  <tr>
  <td>Jack</td>
  <td>Tuesday</td>
  <td>10</td>
  </tr>
  <tr>
  <td>Jack</td>
  <td>Wednesday</td>
  <td>50</td>
  </tr>
  <tr>
  <td>Jill</td>
  <td>Monday</td>
  <td>40</td>
  </tr>
  <tr>
  <td>Jill</td>
  <td>Wednesday</td>
  <td>110</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>becomes:</p>
  <pre class="lang-none prettyprint-override"><code>Jack | Monday     | 10  | 10
  Jack | Tuesday    | 30  | 40
  Jack | Wednesday  | 50  | 90
  Jill | Monday     | 40  | 40
  Jill | Wednesday  | 110 | 150
  </code></pre>
  <p>I tried various combos of <code>df.groupby</code> and <code>df.agg(lambda x: cumsum(x))</code> to no avail.</p>