$bins = int(($keys[$#keys]-$keys[1])/$histowindowsize);

###histowindowsize is some integer (i have tried 1000 and got errors)

my $graph = new GD::Graph::histogram(800,600);
$graph -> set(
x_label => 'Hash's Keys',
y_label => 'count of keys in file',
title => 'Key count histogram',
histogram_bins => $bins,
x_labels_vertical => 1,
bar_spacing => 0,
shadow_depth => 1,
shadowclr => 'dred',
transparent => 0,
)
or warn $graph -> error;
my $gd = $graph -> plot (\@keys) or die $graph->error;
open (IMG, '>histo.png') or die $!;
binmode IMG;
print IMG $gd -> png;