title: Get aggregate sum of data by day and hour
body: <p>The below is an example of the data I have.</p>
  
  <pre><code>     date       time    size    filename            day.of.week
  1   2015-01-16  5:36:12 1577    01162015053400.xml  Friday
  2   2015-01-16  5:38:09 2900    01162015053600.xml  Friday
  3   2015-01-16  5:40:09 3130    01162015053800.xml  Friday
  </code></pre>
  
  <p>What I would like to do is sum up the size of the files for each hour.</p>
  
  <p>I would like a resulting data table that looks like:</p>
  
  <pre><code>date        hour   size
  2015-01-16  5      7607
  2015-01-16  6      10000
  </code></pre>
  
  <p>So forth and so on.</p>
  
  <p>But I can't quite seem to get the output I need.</p>
  
  <p>I've tried ddply and aggregate, but I'm summing up the entire day, I'm not sure how to break it down by the hour in the time column.</p>
  
  <p>And I've got multiple days worth of data. So it's not only for that one day. It's from that day, almost every day until yesterday.</p>
  
  <p>Thanks!</p>