I'm having a problem during fit my data.
I have data for x,y,z and I want find a good correlation for z as a function of x and y. From the data vizualization I can guess that the expression will be z=-ax+by+c. I tried fit the data with this kind of expression and I obtained
"Singular matrix in Invert RtR error during fit"
If I try an expression like z=-x+y+c there are no problems with the data fitting, but the expression predict higher values and not so close to the data as expected. Then I tried adjust by hand the a,b and c parameters and I get a better approximation to the data than the previous.
Why is this happening and how can I solve it?