[x,z] = meshgrid(-2:.5:2,-2:.5:2);
y=-x;
u=ones(size(x));
v=ones(size(y));
w=zeros(size(z));
quiver3(x,y,z,u,v,w)
hold on
surf(x,y,z)
axis([-2 2 -2 2 -2 2])
xlabel('x')
ylabel('y')
zlabel('z')