title: Pandas: subtract one column from another in a pivot table
body: <p>I would like to subtract one columns from another in a pivot table. 'diff' shoud be the difference between 2017 and 2016</p>
  
  <pre><code>raw_data = {'year': [2016,2016,2017,2017],
      'area': ['A','B','A','B'],
      'age': [10,12,50,52]}
  df1 = pd.DataFrame(raw_data, columns = ['year','area','age'])
  
  table=pd.pivot_table(df1,index=['area'],columns=['year'],values['age'],aggfunc='mean')
  
  table['diff']=table['2017']-table['2016']
  </code></pre>