title: gnuplot: how to plot sum of three data sets
body: <p>I have a data in file which I would like to plot using gnuplot. In the file, there are 3 data sets separated by two blank lines so that gnuplot can differentiate between the data sets by 'index'. I can plot three data sets separately via 'index' option of 'plot' command. 
  However, I am not sure how can I plot the data which is sum of 2nd column of all three data sets?  </p>
  
  <p>Note: all three data sets have same x data, i.e. 1st column</p>