title: How to calculate daily sums from a dataframe (options chain data)
body: <p>I'm trying to calculate daily volume sums on a dataframe. The dataframe looks like this (it's an option chain):</p>
  
  <pre><code>     eod_date     expiry type strike  last   bid   ask volume
  1 2015-05-11 2017-01-20 call   65.0  0.00  0.23  0.2      1
  2 2015-05-11 2015-05-15 call   24.0  0.00 14.20 16.00     2 
  3 2015-05-12 2015-05-15 call   27.5  0.00 13.95 15.65     4
  4 2015-05-12 2015-05-15 call   30.0 11.90 11.45 12.05     9
  </code></pre>
  
  <p>The calculated output of daily volume sums should be a dataframe like this:</p>
  
  <pre><code>    eod_date  type volume
  1 2015-05-11 call      3
  2 2015-05-12 call     13
  </code></pre>
  
  <p>I have not been able to figure out a good way to do this. There are a lot of other metrics I'd like to calculate but figuring out this one would be the first step to implementing the others (like volume sums across different expiry dates). Thanks!</p>