Hi,
I guess you create PNG images. If it's so, it is possible that the long time is spent in the compression library that compresses the PNG image data. I encountered that problem once when I created many many images in gnuplot. You can look at that thread here. (And more of it took place in the comp.graph.apps.gnuplot newsgroup ;-)

Maybe for you (as a beginner as you said) it would be wise to stick with the gnuplot program for the start. It is easy to learn/use and you can just open a pipe to it. A quick start: www.gnuplot.org

Actually that is what I usually do; no matter if coding data-stuff in perl or algorithm-ism in C/C++

Update:
1. There is also a way to handle time-data in gnuplot which may come out usefull for your problem
2. I'm not pointing you to any CPAN modules because I think, if you're new to the world of perl you may not want to install your own modules right now (but -of course- you will in -say- 6 months *grin*)

Regards

Stefan K