for loop with parametric plots in gnuplot

Question

I am trying to plot multiple parametric curves in gnuplot 4.6.

In an earlier version (4.4?), the commands

set para

plot [-pi:pi] for [a=1:10] a*cos(t),a*sin(t)

would result in ten circles centered at the origin with radius 1, 2, ..., 10. In 4.6, the result is one circle of radius 1.

In 4.6, the commands

unset para

plot [-pi:pi] for [a=1:10] a*sin(x)

yield ten beautiful sine curves.

So, it appears that the "for" command now has a problem with parametric curve plotting, I guess.

Does anyone know of a workaround? The circle object is not useful for me: I am interested in general curves. Thanks!

That seems to be a bug. It works with 4.4.4, but not with 4.6.0 or the current development version. I reported the bug at sourceforge.net/p/gnuplot/bugs/1314 — Christoph
– Christoph, Commented Jan 3, 2014 at 15:12

Ethan Merritt · Accepted Answer · 2014-01-03 19:10:43Z

3

The syntax ambiguity between parametric mode and iteration is a documented bug/limitation in current gnuplot versions. In the development version (4.7) a separate parametric mode is not necessary, as the required sampling variable can be explicitly described in a generic plot command:

plot for [a=1:10] [t=-10:10] '+' using (a*sin(t)):(a*cos(t))

Unfortunately that fully general syntax is not available in version 4.6. The closest I can think of is a simpler variant:

unset parametric
plot for [a=1:10] '+' using (a*sin($1)):(a*cos($1))

This works for your example case, but may not suffice for your actual use case because it conflates the sampling range on the parametric variable with the implicit plotting range on x.

answered Jan 3, 2014 at 19:10

Ethan Merritt

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1 Comment

Matthew Conroy Over a year ago

Thanks, Ethan. I'll give it a try once I can get 4.7 installed. Cheers!

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