title: gnuplot plot difference of 2 columns
body: <p>I have two files A and B. Both files contain 2 columns, x and y.</p>
  
  <p>Now, I want to plot a graph for x vs (yA - yB). Does gnuplot provide a command for the same ? </p>
  
  <p>One more thing, lets say xA and xB are not same. How should I plot a graph where x-axis contains all elements which are in both, xA and xB and y-axis is the difference is the corresponding y-components ? </p>