If what you're looking for is just a reference line, it'd be pretty easy to hack in, I think. There may be a bug or two in this (though it works in a trivial test case), but I'd think something along these lines would work:

# assuming $graph has already been set up and plotted
my $y_value = 42; # desired y-value for reference line 
my $y_coord = ( $graph->val_to_pixel(0,$y_value,0) )[1];
my $GD = $graph->{graph};
my $red = $GD->colorResolve(255,0,0);
$GD->line($graph->{left},$y_coord,$graph->{right},$y_coord,$red);
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Not that I've been thinking about precisely that problem, recently, or anything like that. ;-)