title: R: Calculating deltas in a timeseries
body: <p>I have a timeseries of samples in R:</p>
  
  <pre><code>&gt; str(d)
   'data.frame': 5 obs. of  3 variables:
   $ date: POSIXct, format: "2010-03-04 20:47:00" "2010-03-04 21:47:00" ...
   $ x   : num  0 10 11 15.2 20
   $ y   : num  0 5 7.5 8.4 12.5
  &gt; d
                   date    x    y
  1 2010-03-04 20:47:00  0.0  0.0
  2 2010-03-04 21:47:00 10.0  5.0
  3 2010-03-04 22:47:00 11.0  7.5
  4 2010-03-04 23:47:00 15.2  8.4
  5 2010-03-05 00:47:00 20.0 12.5
  </code></pre>
  
  <p>In this example samples for x and y are taken every hour (but the time delta is not fix). 
  The x and y values are always growing (like a milage counter in a car). I need the deltas,
  how much was the growth in between, something like this:</p>
  
  <pre><code>1 2010-03-04 20:47:00  0.0  0.0
  2 2010-03-04 21:47:00 10.0  5.0
  3 2010-03-04 22:47:00 1.0   2.5
  4 2010-03-04 23:47:00 4.2   0.9
  5 2010-03-05 00:47:00 4.8   4.1
  </code></pre>
  
  <p>And I also need the deltas per time (x and y delta, divided by the time - delta per hour)</p>
  
  <p>How would I do this in R?</p>