Consider the 3d graph of a quadratic form: 1/2*trans(x)*A*x-trans(b)*x Plot via matlab: surfc(X,Y,Z); add following lines: hold on x=-10:10; y=0.5*(2-3*x); plot(x,y); This plot a straight line which intersects the 3d surface. However, I wish for it be graphed on the contours: i.e. the 3d graph has contours where Z=-200. The plot(x,y) plots the straight line on the z=0 plane. I wish for it graph the straight line on the z=-200 plane. I've considered doing a loops such that I graph single points for the z=-200 plane. Thank you.