How to fit a monotonically increasing map to a certain data?

Assume that I have data, x,y,z all of them are 1xN double. I want to fit a surface with fit([x,y],z,FitType). I want my surface to increase monotonically in x and y. How can I achive this?

Thanks for any advice in advance.

Answer 1

[darova]

Try

f = fit([x(:) y(:)],z(:),'poly11');
% or manually
% f = fit([x(:) y(:)],z(:),'a*x+b*y+c');

[Tokdemir Ibrahim Kerem]

Thank you for your answer. I want to minimize R-square as well. Poly11 will have a very low R-square I guess.