You may have already come across this, but I thought I'd point out the CodeProject article Plot 3D surfaces. I'm not sure if it's feature set is sufficient for your requirements, but it's worth investigating if you haven't already. The obvious advantage is that it's free and open source (so that you might expand it for your particular needs).
Example screenshot:
Another possible alternative that looks reasonably complete is the F# for Visualization library. This is commercial, but by the look of it would do more that what you might need. Don't let the fact that it is designed for F# put you off - you should still to use it directly from C# (or perhaps with a minor amount of interop to make things cleaner). Certainly, the functional programming support offered by C# should make the job relatively easy.
Example screenshot:
(source: ffconsultancy.com)
I've suggested the first (CodeProject) option just in case that happens to suit your requirements, though it may well not. The F# one would almost surely be good enough. Note that F# being a functional language (and hence typically used for mathematical and scientific applications), it is likely to be an excellent option for any functionality related to graph plotting - you may even want to consider writing the portion of program that does plotting in the language, especially if it gets relatively complex.
Hope that helps.
