A slightly different ugly hack, in an attempt to make more use of what GD::Graph provides:

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 coor
+ds of the chart box
$graph->{graph}->line($chosen_x, $ys[0], $chosen_x, $ys[1], 8); # 8 is
+ black, or you could colorAllocate() something
[download]

In my test it seems to line up the vertical better than in tachyon's example, but there may be other reasons for that too. The val_to_pixel() subroutine could be really handy for drawing objects, or inserting bitmaps too, I'll have to play with that some more.

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