I can't reproduce your output, but here is a simplified version of your code that creates an output PNG file for me (if I input some simple numbers in a file):
use strict;
use warnings;
use GD::Graph::histogram;
my $input = 'distance.txt';
unless(open(INPUT, $input)) {
die "\nCannot open input\n";
}
my @data;
while(my $line = <INPUT>) {
chomp $line;
push @data, sprintf ("%.1f\n", $line);
}
my $graph = new GD::Graph::histogram(400,600);
$graph->set(
y_label => 'Count',
x_label => 'Distances',
title => 'Histogram',
x_labels_vertical => 1,
bar_spacing => 0,
shadow_depth => 1,
shadowclr => 'dred',
transparent => 0,
)
or warn $graph->error;
my $image = $graph->plot(\@data) or die $graph->error;
open( IMG, '>' . 'out.png' ) or die $!;
binmode IMG;
print IMG $image->png;
Post a few lines of your input file (inside "code" tags).
Your $filename variable is undefined because you didn't pass any arguments to your "main" sub. @_ holds args passed to a sub. Also, you keep clobbering your @data when you assign it to @_ (which is empty).