title: Percentage change with groupby python
body: <p>I have the following dataframe:</p>
  
  <pre><code>Year    Month   Booked
  0   2016    Aug 55999.0
  6   2017    Aug 60862.0
  1   2016    Jul 54062.0
  7   2017    Jul 58417.0
  2   2016    Jun 42044.0
  8   2017    Jun 48767.0
  3   2016    May 39676.0
  9   2017    May 40986.0
  4   2016    Oct 39593.0
  10  2017    Oct 41439.0
  5   2016    Sep 49677.0
  11  2017    Sep 53969.0
  </code></pre>
  
  <p>I want to obtain the percentage change with respect to the same month from last year. I have tried the following code:</p>
  
  <pre><code>df['pct_ch'] = df.groupby(['Month','Year'])['Booked'].pct_change()
  </code></pre>
  
  <p>but I get the following, which is not at all what I want:</p>
  
  <pre><code>Year    Month   Booked  pct_ch
  0   2016    Aug 55999.0 NaN
  6   2017    Aug 60862.0 0.086841
  1   2016    Jul 54062.0 -0.111728
  7   2017    Jul 58417.0 0.080556
  2   2016    Jun 42044.0 -0.280278
  8   2017    Jun 48767.0 0.159904
  3   2016    May 39676.0 -0.186417
  9   2017    May 40986.0 0.033017
  4   2016    Oct 39593.0 -0.033987
  10  2017    Oct 41439.0 0.046624
  5   2016    Sep 49677.0 0.198798
  11  2017    Sep 53969.0 0.086398
  </code></pre>