title: pandas calculate variance from aggregation
body: <p>I have a data frame with these columns: <code>Date</code>, <code>ID</code>, and <code>Value</code>. And I need to perform mean, median and variance on <code>Value</code> and I used <code>.agg</code> like this:</p>
  
  <pre><code>df = dataset\
      .groupby(['ID', pd.Grouper(key='Date', freq='60T')])['Value']\
      .agg(['mean', 'median', 'var'])\
      .reset_index()
  </code></pre>
  
  <p>It successfully calculates mean, but when it need to calculate median it simply repeats the mean value and doesn't store or create the var column. Here's the result:</p>
  
  <pre><code>      ID                 Date      mean    median  var
  0  13834  2017-02-09 12:00:00  1.474920  1.474920  NaN
  1  13834  2017-02-09 16:00:00  4.424796  4.424796  NaN
  2  13834  2017-02-09 20:00:00  2.241871  2.241871  NaN
  3  13834  2017-02-10 00:00:00  2.654867  2.654867  NaN
  4  13834  2017-02-10 04:00:00  2.654867  2.654867  NaN
  5  13834  2017-02-10 08:00:00  0.511062  0.511062  NaN
  </code></pre>
  
  <p>At the end of the last number there should be the variance column, instead I get nothing (or <code>NaN</code>s, if shown in a data frame). How do I fix this?</p>