I think you need to distinguish between linear functions and linear models. Polynomials are nonlinear functions but are linear models - Y=f(X,X2, X3, etc.) can be a linear function. Fit curve will fit such polynomials. It will also fit things that are not linear models. So, you can confine yourself to the polynomials for a start. Now, if you want logarithmic transformations, then I think you will need to follow txnelson's advice and create columns for these. Of course, a log transformation is a nonlinear function, but you can have a linear model in the logs of X and/or Y. No platform will automatically cover all potential transformations. I don't think you are insisting on only linear functions - least square regression will give you the best fitting linear function and there is no need to try others since the regression line (surface) is already the best fitting one.