CODEWITHSUNDEEP
gnuplot
Martin
Martin

Reputation: 381

How to use multiple lines of a data file in gnuplot?

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?

Upvotes: 2

Views: 442

Answers (2)

mata
mata

Reputation: 69092

To do this in gnuplot directly is tricky. The problem is that you have to use the (i-1)th element in your calculation, which gnuplot can't do automatically. It can do simple calculations, but only on the same row, for example something like

plot "datafile" using ($1):(($2-$1)/$2)

would be easy.

For what you neet to do I'd recommend octave, or you could prepare your data file using a spreadsheet application.

In octave, you could plot this like:

d = load("datafile")
plot(d(2:end, 1), d(1:end-1, 2)-d(2:end, 2)./d(1:end-1, 2))

Upvotes: 0

mgilson
mgilson

Reputation: 310287

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.

Upvotes: 2

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