I think the reason that the graph is not quite right is that you are appending to @data on each pass through the loop, but you don't reinitialize it, so on the second pass @data has both sets of data in it. I assume that GD is ignoring the extra set.

I would suggest that you do all of the following as a matter of course:

(1) use warnings;

(2) use strict;

(3) add "my" where needed to declare variables in the correct scope (you won't need to explicitly reinitialize @data if it is made local to the loop), and finally

(4) run the script "perltidy" from the CPAN module Perl::Tidy on your code to fix the indentation. This isn't just esthetic, it can make flaws in scoping and nesting a lot more obvious. (Update: a better description of using command-line perltidy is at http://perltidy.sourceforge.net/.)