# 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);