How can I plot this?

Question

Given a bunch of data $x_i$ , $y_i$, how do I plot $$f(\theta_2,\theta_2)= \frac{1}{2M} \sum_{i=1}^{M} (\theta_1\cdot x_i -\theta_2 y_i)^2$$ in matlab? I know it should be parabolic, but my code (shown below) doesn't give me a parabaloid.

x=linspace(0,1,1001);
M=numel(x);

y=2*x-3+(-2+4*rand());

plot(x,y)

[theta1,theta2]=meshgrid(0:0.01:3,-5:0.01:-2);

J=zeros(length(theta1),length(theta2));

for i = 1:M

     J=J+(theta1*x(i)-theta2-y(i)).^2;

 end

 J=(2*M)^(-1)*J;

 mesh(theta1,theta2,J)

Answer 1

I changed the domain of theta1 and theta2 to

[theta1,theta2]=meshgrid(-100:0.5:100,-100:0.5:100);

and I got the following parabaloid-looking plot: