Use variable expansion in heredoc while piping data to gnuplot

Question

I normally use a code like following to pipe data from a file to gnuplot and create a picture during the Perl script:

#!/usr/bin/perl
use warnings;
use strict;

my $in="file.dat";

open(GP, "| gnuplot") or die "$!\n";
print GP << "GNU_EOF";

set terminal png size 1920,1080 font 'Verdana,15' dashed
set output 'out.png'
plot "$in"

GNU_EOF

close(GP);

I have to define "GNU_EOF" instead of 'GNU_EOF' so I can use variables like $in.

Now I want to use data which isn't read from a file directly. My code looks like:

#!/usr/bin/perl
use warnings;
use strict;

open(GP, "| gnuplot") or die "$!\n";
print GP << 'GNU_EOF';

set terminal png size 1920,1080 font 'Verdana,15' dashed
set output 'out.png'
plot '-'

GNU_EOF

open(INFILE,"< stuff.dat") or die "$!\n";
while (my $line = <INFILE>) {

for my $i (1..10){
    # do some stuff to calculate my data points stored in $x and $y
    print GP "$x $y\n";
}
print GP "EOF\n";
}

close(INFILE);
close(GP);

If I try this using "GNU_EOF" to be able to define variables in the heredoc, I am getting errors like:

gnuplot> 187 0.05
         ^
         line 1: invalid command

I don't know

why I have to use "" for the heredoc to get the desired variable expansion and
why I get errors for the second example.

Help is highly appreciated.

My guess is that you need to pass something more than just the two coordinates to gnuplot. What does file.dat look like in your first example? — Dave, Dec 06 '13 at 14:44
For me your example works fine without the line `print GP "EOF\n";` and with `pngcairo` (instead of `png` because of the `dashed` terminal option). — Christoph, Dec 06 '13 at 14:48
@Christoph, You misread what I wrote because I fully know the error from was `gnuplot`. Anyway, I've already removed my comment for other reasons. — ikegami, Dec 06 '13 at 15:16
@Christoph I need the line `print GP "EOF\n";` because I am using several loops to get data points. Nonetheless I tried it without this line (and only with one for-loop) which isn't solving the error. Neither pngcairo is a problem (I am also using pdf output, same error here). — EverythingRightPlace, Dec 06 '13 at 15:17
@Christoph, The `print GP "EOF\n"` should not be removed. It tells `gnuplot`'s `plot` when to stop reading from `-`. (My version prompts me to use `e`, but `EOF` apparently works too.) — ikegami, Dec 06 '13 at 15:17
The [Chart::Gnuplot](https://metacpan.org/pod/Chart::Gnuplot) module provides (incomplete) bindings to the gnuplot program, which might be preferable to templating the commands yourself. — amon, Dec 06 '13 at 15:18
Correct, the line `print GP "EOF\n";` terminates `plot '-'`. So all the data which is sent after that, when the `while` loop continues, is interpreted as new commands. And gnuplot doesnt know commands like `187 0.05`. — Christoph, Dec 06 '13 at 15:24
@Christoph Nothing is piped to Gnuplot during the `while` loop after the `print GP "EOF\n";` line. I set this line because otherwise the `print GP "$x $y\n"` wouldn't have an ending and Gnuplot wouldn't be able to handle upcoming numbers. Please see my answer I posted. I am looking forward to hear your thoughts because I don't completely understand it. — EverythingRightPlace, Dec 06 '13 at 15:41
I thought you had a data file with several lines which you were going through. And as soon as you have more than one line in your `stuff.dat` you'll get the error with the script you have posted in the question. That shows the importance of minimal, but complete example ;) You were missing the content of `stuff.dat`, and the calculations of `$x` and `$y`, which made it difficult to track down your real problem (at least for me). — Christoph, Dec 06 '13 at 15:51
@Christoph you are right, sorry for the bad example. I just didn't thought this would cause any problems. My `stuff.dat` has just one line with lots of columns. I want to plot several lines/graphs (using several `plot`) and therefore I have to re-structure the data. Nonetheless I have no clue why the line-break is problematic. — EverythingRightPlace, Dec 06 '13 at 15:59
Maybe you need to escape the backslash as `\\ ` for use with `"GNU_EOF"`. — Christoph, Dec 06 '13 at 16:43

score 0 · Accepted Answer · answered Dec 06 '13 at 15:36

I solved it. Sorry for the incomplete question, I wanted to give a minimal example to avoid confusion. Unfortunately my question missed the important part. Like said in the comments I use several loops to generate data and pipe it to gnuplot:

#!/usr/bin/perl
use warnings;
use strict;

open(GP, "| gnuplot") or die "$!\n";
print GP << 'GNU_EOF';

set terminal png size 1920,1080 font 'Verdana,15' dashed
set output 'out.png'
plot    '-' t "one", \
        '-' t "two"

GNU_EOF

open(INFILE,"< stuff.dat") or die "$!\n";
while (my $line = <INFILE>) {

for my $i (1..10){
    # do some stuff to calculate data points stored in $x and $y
    print GP "$x $y\n";
}
print GP "EOF\n";
}

for my $i (1..50){
    # do some other stuff to calculate data points stored in $x and $y
    print GP "$x $y\n";
}
print GP "EOF\n";
}

close(INFILE);
close(GP);

I don't know why, but using 'GNU_EOF' (without variable expansion) I can define several plot with the line brake command \.

Using "GNU_EOF" I have to define it in one line:

plot '-' t "one", '-' t "two"

Sorry for the struggle, but maybe this is also helpful for someone else (and maybe you can explain this behaviour to me).

`<<"X"` is like `"..."` and `<<'X'` is close to `'...'`. If you want to include a slash in a double-quoted string, you must escape it. — ikegami, Dec 06 '13 at 17:40

Use variable expansion in heredoc while piping data to gnuplot

1 Answers1