Is there any IDL function available, which performs a surface fit to a 2-dim array with different polynomial degrees in x and y direction? That means, the routine should work like the SFIT function, but with 2 types of polynomial degrees: one degree for the x direction and the other one for y direction. E.g. degree 3 (cubic) in x direction and degree 2 (parabolic) in y direction. Ideally the polynomial fit function could also be applied to irregular data (see SFIT keyword "/irregular".). The result should be either the fit coefficients or the fit array.