As an after thought, considering the hubris and ingenuity of perl programmers, there may be a way to hack a GD solution, by stitching together 2 GD images. The first, would be your normal graph, and the second would be just an custom image containing your second y axis, which you would overlay and stitch to the right side of the actual graph.

I would look at the demo pages for each program, like gnuplot demo page and see if you find a model you like, especially on gnuplot axis tic demo

It looks like they call the second independent y-axis a "z-range".