Surface plot of a function of two variable.
z An mXn matrix of the function values to plot.
x Either a vector of length n or a matrix of size mXn .
y Either a vector of length m or a matrix of size mXn .

The inputs x and y should either be both vectors, or matrices of the indicated size. In the former case, the function is assumed to be of the form z[i,j]=f(x[j], y[i]) . Otherwise, the functional form z[i,j]=f(x[i,j],y[i,j]) is assumed.

Each region defined by x[i,j]<=x<=x[i,j+1], y[i,j]<=y<=y[i+1,j] is filled with the same color, which is determined by the average z value in this region. Further, the boundaries of of each of these regions are drawn in black.
>>title('Surface Plot')