title: Groupby mean ignoring zero
body: <p>I have a pivot table which has this structure :</p>
  <pre><code>        correl
          stock_b      AAAA    CCCC    DDDD
  stock_a date            
  AAAA    2018-01-02   0.00    0.00    0.00
          2018-01-10   0.00    0.00    0.00
          2018-01-15   0.00   69.45   69.14
          2018-01-31   0.00    0.00    0.00
  BBBB    2018-01-02   0.00   43.29   67.38
          2018-01-14   0.00    0.00    0.00
          2018-01-15  50.16   42.74   67.47
          2018-01-31   0.00   42.74   67.47
  </code></pre>
  <p>I would like to group by <code>stock_a</code> and take the mean value, ignoring zero. It would look like :</p>
  <pre><code>         correl
  stock_b  AAAA    CCCC    DDDD
  stock_a         
  AAAA     0.00   69.45   69.14
  BBBB    50.16   42.92   67.44
  </code></pre>
  <p><code>df.groupby('stock_a').mean()</code> take in account the 0 and the value is not what i would like to have.</p>