3D-Plot of a portfolio variance

Hello,

I would like to create a 3D-plot of the variance of a portfolio of two assets.

More precisely, let V denote the (2x2) variance covariance matrix and X denote the (2,1) vector of weights. The portfolio variance is then defined by the following matrix multiplication: X' * V * X

What I want to do is calculate the portfolio variance for all possible Xs, where the elements of X vary from -1 to +2 (-1:0.01:2).

I have already implemented a code:

a1 = [-1:0.01:2].';

a2 = [-1:0.01:2].';

b = zeros(length(a1),length(a1));

for i=1:length(a1);

for j=1:length(a1);

X=[a1(i,1); a2(j,1)];

b(j,i) = X.'*V*X;

end

end

This code runs, but it is computationally too expensive. Any ideas on how to implement this problem in a feasible way?

Thanks.