If I understand your question, you have two input variables, P and n, and a result e = e(P,n). So you create a grid of values for P and n using meshgrid, then calculate e for each pair P(i),n(j). If the resulting error is a smooth function of the inputs, you could plot it using
surf(P,n,e)
or use contour. If it's not so smooth, you might prefer to use imagesc.
EDIT: To clarify, you need a loop something like this:
p = 5:5:70;
n = 20:20:200;
[P,N] = meshgrid(p,n);
e = 0*P; % just an initialization
for i=1:numel(P)
e(i) = simulateAndReturnError(P(i),N(i));
end
surf(P,N,e) % or imagesc(p,n,e)
