title: How to find the max absolute value for each level of a multi-index DataFrame
body: <p>How to find the max absolute value for each level of a multi-index data frame?</p>
  
  <p>Here's how the data looks like:</p>
  
  <pre><code>                        data
              a   b   c   
              1   X   1   -2
                  X   2   +2
              2   X   1   -1
                  X   2   +2
                  Y   1   +6
              3   X   1   -5
                  Y   1   -3
                  Y   2   +5
  </code></pre>
  
  <p>Here's what I expect to receive while keeping level 1 and 2 of the multi-index:</p>
  
  <pre><code>                        data
                  a   b   
                  1   X   2
                  2   X   2
                      Y   6   
                  3   X   -5
                      Y   +5
  </code></pre>