n = 101;

Q = [-1 0 ; 0 1];
[x1,x2] = meshgrid(linspace(-1.5,1.5,n));
y = zeros(size(x1));

for i = 1:n
   for j = 1:n
      y(i,j) = [x1(i,j); x2(i,j)]'*Q*[x1(i,j); x2(i,j)];
   end
end
figure(1);clf;
surf(x1,x2,y);
shading interp