Octave won't plot 3D graphs anymore.

[Eusebio, Paulo]

I have GNU Octave version 3.2.4. It used to be able to plot 3D graphs but not anymore. Everytime I try to run the following program it keeps saying 'invalid use of script in index notation'. What does that mean? The following code is thusly:

% mesh.m

% produces 3-D graph of analytic solution to laplace equation

clear;

a = -2; b = 2; N = 100; h=(b-a)/N;

xx = a:h:b; yy=xx;

[x,y]=meshgrid(xx,yy);

z = 1./(1+(x.^2 + 2*x.*y + y.^2));

figure;

surf(x,y,z);

axis([a,b,a,b,0,1]); xlabel("x"); ylabel("t")

How do I fix this problem? Do I have to upgrade or delete certain libraries?