title: pandas: Grouping by two columns and then sorting it by the values of a third column
body: <p>I have the following line:</p>
  
  <p><code>genre_df.groupby(['release_year', 'genres']).vote_average.mean()</code></p>
  
  <p>This gives me the following:</p>
  
  <pre><code>release_year  genres         
  1960          Action             6.950000
                Adventure          7.150000
                Comedy             7.900000
                Drama              7.600000
                Fantasy            7.300000
                History            6.900000
                Horror             8.000000
                Romance            7.600000
                Science Fiction    7.300000
                Thriller           7.650000
                Western            7.000000
  1961          Action             7.000000
                Adventure          6.800000
                Animation          6.600000
                Comedy             7.000000
                Crime              6.600000
                Drama              7.000000
                Family             6.600000
                History            6.700000
                Music              6.600000
                Romance            7.400000
                War                7.000000
  ...
  </code></pre>
  
  <p>What I'd like to see is the df grouped by release year and genre, but sorted by the highest vote average first.</p>
  
  <p>AKA:</p>
  
  <pre><code> release_year  genres         
      1960          Horror             8.000000
                    Comedy             7.900000
                    Action             6.950000
                    Thriller           7.650000
                    Drama              7.600000
                    Romance            7.600000
                    Fantasy            7.300000
                    Science Fiction    7.300000
                    Adventure          7.150000
                    Western            7.000000
                    History            6.900000
  </code></pre>
  
  <p>How can this be achieved?</p>