I can't be certain, but I suspect the problem is in the last print statement you show.
print $my_graph->plot(\@data)->$format();
The plot function returns a GD object, for example:

my $gd = graph->plot(...);
my $format = $my_graph->export_format; 
print header("image/$format"); 
binmode STDOUT;
print $gd->gif;
[download]