Your immediate problem is that the line:

    $graphimage = $graph->plot(\@data) or die $graph->error;
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in sub plot replaces the object you passed in. The easiest way to handle the issue is to simply return the object from plot. You don't need to pass anything in:

my $g = plot ();
...

sub plot {
    my @data = (
    ...
    ) or die $graph->error;

    return $graph->plot(\@data) or die $graph->error;
}
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