title: renaming columns after group by and sum in pandas dataframe
body: <p>This is my group by command:</p>
  
  <pre><code>pdf_chart_data1 = pdf_chart_data.groupby('sell').value.agg(['sum']).rename(
      columns={'sum':'valuesum','sell' : 'selltime'}
  )
  </code></pre>
  
  <p>I am able to change the column name for value but not for 'sell'.
  Please help to resolve this issue.</p>