X    Y
1    10
2    20
3    30  ETC

##</code><code>##

open (FH, "<data.txt");

while (<FH>) {
chomp ($_);

($x, $y) = split (/\s+/, $_) unless /RT/;
push @array, ($x, $y); 
              }


my $mygraph = GD::Graph::lines->new(600, 300);
$mygraph->set(
x_label => 'RT',
y_label => 'HEIGHT',
title => 'xxx'
);

$image = $mygraph->plot(\@array) or die $mygraph->error;

open(IMG, '>file2.png') or die $!;
binmode IMG;
print IMG $image->png;
close IMG;