title: Order of summarize and diffSeries with graphite
body: <p>I'm trying to subtract the sum of a couple graphite metrics from the sum of a couple of other graphite metrics over a summarized period of time.  However, I'm getting different results depending on whether I summarize before or after I do diffSeries.</p>
  
  <p>I expect these two graphite queries below to be the same, but they're not:</p>
  
  <pre><code>summarize(
    diffSeries(
      sum(
        stats_counts.otherthing.one,
        stats_counts.otherthing.two
      ),
      sum(
        stats_counts.thing.one,
        stats_counts.thing.two,
        stats_counts.thing.three
      )
    ),
    '1week',
    'sum',
    true
  )
  </code></pre>
  
  <p>With summarize on the outside of diffSeries (as above) I get numbers that seem way too high based on looking at the raw data in csv exports.</p>
  
  <pre><code>diffSeries(
    summarize(
      sum(
        stats_counts.otherthing.one,
        stats_counts.otherthing.more_than_five
      ),
      '1week',
      'sum',
      true
    ),
    summarize(
      sum(
        stats_counts.thing.one,
        stats_counts.thing.two,
        stats_counts.thing.three,
      ),
      '1week',
      'sum',
      true
    )
  )
  </code></pre>
  
  <p>The above seems to give me the right data, but I'd rather only have to summarize once and can't for the life of me figure out why it should be different.  It seems like the arguments to diffSeries are getting messed up when I just pass them in as sums and don't summarize too, but I'm not sure how.</p>