Please check the following:

1. Type gnuplot --version to check whether gnuplot can be executed.
2. Check whether the directory /tmp is already full.
3. Check whether you have write permission of /tmp.

If they are all fine, then you should be able to use at least the basic features of Chart::Gnuplot. Please re-try your original script with expression.ps as output instead (ps format is more basic than png for the module).

You don't need the method ->output($filename) although there is no harm to do so. The output method is useful only in cases where you don't know the output filename yet when you create the chart object.

If you got expression.ps, then it may be the problem of ImageMagick. Please check whether it is installed.

If you don't want to install ImageMagick and if you want to generate png file. You may try:

my $chart = Chart::Gnuplot->new(
output => "/home/diffredential/Desktop/expression.png",
terminal => "png"
);
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