my $chosen_x = ($graph->GD::Graph::axestype::val_to_pixel(5,0,1))[0]; # only the 5 matters here my @ys = @{$graph->get_feature_coordinates('axes')}[2,4]; # the y coords of the chart box $graph->{graph}->line($chosen_x, $ys[0], $chosen_x, $ys[1], 8); # 8 is black, or you could colorAllocate() something