If I may add a little weight on that matter, and I know it has been asked for earlier on these forums, I'd also appreciate a second Y axis so I could compare two point-series which aren't the same scale.
My use of that is for engineering and simulation projects, mainly comparing trends of a control variable on simulation result variable. I'd also use it with logged data, I'd like to easily see the simultaneity of changes on two logged points of measurement in the same graph.
I understand that using two y-axes is a major change to all the function names and how you refer to them when programming Graph. I don't think you need to generalize your code to "n" y-axes, two is enough for most needs I know of (I don't know everything), although it would not be modular.
Thanks!