DPlot

does Dplot incorporate least-squares fitting of curves to find gradients etc?

Yes. See Generate>Polynomial Curve Fit. Or with the licensed version also see Generate>More Curve Fits, which also includes a least-squares polynomial fit but has a few more options, including the ability to fit a range of your data and hold one or more coefficients in the resulting equation to fixed values.

I am sure somebody has requested this already, but if you could get someone to help you with the code for generalized curve fitting (for an arbitrary function), I would no longer even bother buying a program like Origin or Sigmaplot, because then I could do virtually everything I need with Dplot and Excel put together

I've started down that road several times, but all of my efforts to date end up with too many instabilities - you basically have to have a pretty good idea of the answer before you ask the question, else the solution procedure blows up with an overflow or other math error.

But thanks for the reminder.