title: Can gnuplot compute and plot the delta between consecutive data points
body: <p>For instance, given the following data file (x^2 for this example):</p>
  
  <pre><code>0
  1
  4
  9
  16
  25
  </code></pre>
  
  <p>Can gnuplot plot the points along with the differences between the points as if it were:</p>
  
  <pre><code>0 0
  1 1   # ( 1 - 0 = 1)
  4 3   # ( 4 - 1 = 3)
  9 5   # ( 9 - 4 = 5)
  16 7  # (16 - 9 = 7)
  25 9  # (25 -16 = 9)
  </code></pre>
  
  <p>The actual file has more than just the column I'm interested in and I would like to avoid pre-processing in order to add the deltas, if possible.</p>