title: Avoid duplicate count columns with pandas groupby
body: <p><code>pandas.Dataframe.groupby(['date','some_category']).agg([np.sum, np.size])</code> produces a count that is repeated for each sum column. Is it possible to output just a single count column when passing a list of aggregate functions?</p>
  
  <pre><code>a = df_all.groupby(['date','some_category']).sum()
  b = df_all.groupby(['date','some_category']).size()
  pd.concat([a,b], axis=1)
  </code></pre>
  
  <p>produces basically what I want but seems awkward.</p>