title: plot function of datasets from different files in gnuplot
body: <p>I have several files with similar data that I need to plot using gnuplot.</p>
  
  <p>By example, I use something like this to plot the 1st Vs 5th column of 3 files:</p>
  
  <pre><code>plot "file1.csv" using 1:5 title 'test 1' with lp, \
       "file2.csv" using 1:5 title 'test 2' with lp, \
       "file3.csv" using 1:5 title 'test 3' with lp
  </code></pre>
  
  <p>but, I don't know how to plot a function of the data from the 3 files. By example, I would like to include in the previous plot the media of the 3 columns for each point (that would be the function <code>f(x1,x2,x3)=(x1(i)+x2(i)+x3(i))/3</code> for the ith data point). Is it possible?</p>