title: How can use pandas group-by while adding new columns on a single level?
body: <p>The original data looks like this: </p>
  
  <pre><code>    Date        E   
  0   2017-09-01  -   
  1   2017-09-01  +   
  2   2017-09-01  +   
  3   2017-09-01  +  
  ...
  ... 
  </code></pre>
  
  <p>After applying groupby:</p>
  
  <pre><code>df.groupby(['Date', 'E'])['Date'].count().to_frame(name = 'Count').reset_index()
  </code></pre>
  
  <p>I get a dataframe that looks like this:</p>
  
  <pre><code>    Date        E   Count
  0   2017-09-01  +   11
  1   2017-09-01  -   1
  2   2017-09-04  +   1
  3   2017-09-04  -   7
  4   2017-09-05  +   1
  5   2017-09-05  -   23
  </code></pre>
  
  <p>How can I transform this into a dataframe that instead looks like this:</p>
  
  <pre><code>    Date        +   -
  0   2017-09-01  11  1
  2   2017-09-04  1   7
  4   2017-09-05  1   23
  </code></pre>