My prior reply was from work at my new job. This addenda is from home. I have searched the home PC for my last communication with Martien Verbruggen to no avail. I had hoped to quote verbatim from that exchange. But I fear it must have been from work at my old job.
I've actually been waiting for multi-data set graphs to work in dual Y-axis line graphs for more than three years. They sort of work now but the Y2 tick marks often times get out of control. And results are unprofessional until that gets sorted out, which I'm sure Mr. Verbruggen will get to eventually. But it is not top of his list, this being a rarely used feature. Meanwhile I needed something now. I'm not trying to compete, just share what I've got...such as it is.
On the other hand, I don't very much like how GD::Graph::lines force-aligns both Y1 and Y2 to share a common zero because it scrunches the vertical resolution should all data sets for a given Y axis be on same side of zero. It is this very same beef with respect to another software product in 1992 that spurred me to write the original PostScript program which I grafted piecemeal to Perl for this present module.
I'm not bucking GD::Graph in general. It's wonderful for every other use except this one, admittedly unusual, case in particular. My cool use for Perl was only to resurrect an otherwise user-unfriendly PostScript program to temporarily solve a vexing graph-related roadblock so that I could get on with a project.