Question

I have a data file with two columns: Xi and Yi. I’d like to plot Xi vs. (Yi-1 - Yi)/Yi-1 for i>1. Is is possible to do that in GNUPlot direclty?

Answer 1

Yes it is possible with gnuplot directly — It’s just not easy:

firstval = NaN
yi1(yi) = (returnval=firstval, firstval=yi, returnval)
plot "datafile" using 1:((yi1($2)-$2)/returnval)

You need to use inline functions. inline functions are of the form:

funcname(args,...) = (statement1,statement2,...,statementn, return_value)

Here I just created a function to hold the last value it was passed. Unforunately, this solution gets a little more ugly since I couldn’t call yi1 twice in the using specification (the second time, I would get the wrong return value), so I had to reference the variable holding the return value directly. It’s not pretty, but it works. You could probably “pretty” it up a little bit by passing $0 (the line number) and only updating when $0 changes, but it’s probably not worth it for this hack.