AFAIK there is no *method* but there is a *way* It does involve tinkering directly with the object internals and a private method but it gets the job done. See this example png from the code below Any direct tinkering with private methods is to be frowned upon but it might be a worthwhile patch to submit for a new method....

use GD::Graph::lines;

my @data = ( 
    ["1st","2nd","3rd","4th","5th","6th","7th", "8th", "9th"],
    [    1,    2,    5,    6,    3,  1.5,    1,     3,     4],
    [ sort { $a <=> $b } (1, 2, 5, 6, 3, 1.5, 1, 3, 4) ]
);

my $graph = GD::Graph::lines->new(400, 300);

$graph->set( 
    x_label           => 'X Label',
    y_label           => 'Y label',
    title             => 'Some simple graph',
    y_max_value       => 8,
    y_tick_number     => 8,
    y_label_skip      => 2 
) or die $graph->error;

# plot it to get some of the data values we want into $graph object
my $gd = $graph->plot(\@data) or die $graph->error;

# you may need to look at the object if your line pos is less simple
#use Data::Dumper;
#print Dumper $graph;

# this is the hack.....
my $x_zero = $graph->{left};         # abs zero point co-ord
my $x_step = $graph->{x_step};       # x step pixels
my $xp = $x_zero + $x_step * 5;      # the 5th data point abs x co-ord
my $t = $graph->{top};               # top abs y co-ord
my $b = $graph->{bottom};            # bottom abs y co-ord
my $mclr = 2; # 1 = blue, 2 = grey, 3 = red ...
# now draw the line in......
$graph->{graph}->line($xp, $t, $xp, $b, $mclr);

open(IMG, '>C:/file.png') or die $!;
binmode IMG;
print IMG $gd->png;
close IMG;
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