how to automatically sort the x values in gnuplot

Question

When I try to plot something with linespoints, unless the values that go to the x axis are already sorted within the file, each point gets connected to the point that is on the next line of the file:

enter image description here

Only when I sort the values in the file I can get the desired effect, which is that each point gets connected to the point with one smaller and one larger x value:

enter image description here

Is there a way to do this within gnuplot, without having to sort the files in bash?

gnuplot will probably never implement a "real" internal sort feature. I guess the idea is that you are supposed to use existing tools e.g. sort under Linux (for Windows you have to install comparable tools, e.g. gnuwin32.sourceforge.net/packages/coreutils.htm). There are cumbersome attempts with gnuplot-only, e.g. stackoverflow.com/q/54144027/7295599. Although, in gnuplot 5.4 there is zsort which, however, is not maintaining the order in a two column sort (however only under Windows), see: stackoverflow.com/q/67801386/7295599 — theozh
– theozh, Commented Aug 11, 2022 at 18:58

Christoph · Accepted Answer · 2015-05-25 21:34:58Z

20

Gnuplot offers some smoothing filters which as first step sort the data by their x-values. plot ... smooth unique first sorts the data points by their x-value and for equal x-values it computes the average y-value. So if you are sure that the x-values are unique, then you can use this option. Otherwise you must use an external tool or script to do the sorting with plot '< sort file.dat'

answered May 25, 2015 at 21:34

Christoph

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2 Comments

arekolek Over a year ago

plot '< sort -nk2 file.dat' u 2:3 will use a numerical sort on column 2. This way the header will stay in its place, and you can use choose the column for x-values.

Eric Duminil Oct 13 at 9:05

Just in case it helps someone: I needed to sort by Y values and make sure scientific notation was properly parsed: plot '< LC_NUMERIC=US_en sort --general-numeric-sort --key 2 --reverse "file.dat"' using 0:2. It's useful in order to display en.wikipedia.org/wiki/Load_duration_curve

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