title: Subtract and divide dataframes with different length, but only show the ones with changed value
body: <p>I have two dataframes with different row lengths, similar columns but not arranged accordingly. I would like to subtract df2 from df1 and then divide the result by df1 (to basically get a percentage at the end). </p>
  
  <p>Df1</p>
  
  <pre><code>Index       Bircher  Club   Quiche
  2019-1-18     16       1     4
  2019-1-19     4        9     6
  2019-1-28     8        1     6
  2019-1-29     4        7     6
  2019-1-20     8        1     6
  </code></pre>
  
  <p>Df2</p>
  
  <pre><code>Index       Bircher  Quiche  Club
  2019-1-10     15       1     3
  2019-1-18     4        1     1
  2019-1-20     4        2     6
  2019-1-26     2        1     5
  2019-1-25     4        1     2
  </code></pre>
  
  <p>Outcome: Only the date 2019-1-18 and 2019-1-20 show up in both dataframes and can be used. Then Df3 is simply expressed with this math formula (Df1-DF2)/Df1</p>
  
  <pre><code>Index       Bircher  Club   Quiche
  2019-1-18     75%     0%     75%
  2019-1-20     50%    -100%    0%
  </code></pre>