title: Gnuplot filledcurves with palette
body: <p>I have been trying to change the fillstyle for the filledcurves option in gnuplot so that the fill colour represents the difference between the two curves on a 2-dimensional plot. I am thinking of this as an extension of the 'filledcurves above/below' option whereby instead of just having two colours representing above or below there is a colour range or palette.</p>
  
  <p>Here is an example of the plot I would like to make from a datafile using the above/below filled curve style. A colourbar representing the y-difference between the two curves would be very useful.</p>
  
  <p><img src="https://i.stack.imgur.com/RLlpU.png" alt="http://i.stack.imgur.com/RLlpU.png"></p>
  
  <p>I have tried to do this by adding a fourth column to the using command i.e.</p>
  
  <pre><code>plot 'data.txt' using 1:2:3:($3-$2) with filledcurves fs palette
  </code></pre>
  
  <p>but the <code>filledcurves</code> does not appear to accept the fourth column... I have also considered trying rgb variable but this doesn't seem to work either.</p>