title: pandas moving average by group calculation is wrong
body: <p>My dataframe priceDF begins like this:</p>
  <pre><code>    Date        symbol  Close
  0   2000-01-03  HELN.SW 28.28
  1   2000-01-04  HELN.SW 27.50
  2   2000-01-05  HELN.SW 26.71
  3   2000-01-06  HELN.SW 27.16
  4   2000-01-07  HELN.SW 27.50
  </code></pre>
  <p>To calculate the moving average of the Closes, grouped by symbol I did:</p>
  <p><code>priceDF['avg'] = priceDF.groupby('symbol')'Close'].rolling(3).mean().reset_index(drop=True)</code></p>
  <p>and I get:</p>
  <pre><code>    Date        symbol  Close   avg
  0   2000-01-03  HELN.SW 28.28   NaN
  1   2000-01-04  HELN.SW 27.50   NaN
  2   2000-01-05  HELN.SW 26.71   12.537398
  3   2000-01-06  HELN.SW 27.16   12.022164
  4   2000-01-07  HELN.SW 27.50   11.922733
  
  </code></pre>
  <p>In row 2, I want avg = 27.50, the average of the Closes in rows 0 to 2.
  What do I misunderstand?</p>