open (FH, "<data.txt");
while (<FH>) {
   chomp;
   next if /RT/;
   ($x, $y) = split(/\s+/, $_);
   push @{$array[0]}, $x;
   push @{$array[1]}, $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;