How about the following code. This code plots 'a' and 'b' as a function of 'y' where 'x' = 1 (constant).
A = [2 1;1 -30];
xy = [ones(1,11); 0:1:10];
ab = zeros(size(xy));
for kk = 1:11
ab(:,kk) = inv(A)*xy(:,kk)
end
figure
plot(xy(2,:),ab(1,:),xy(2,:),ab(2,:))
title('a,b as a function of y (where x = 1)')
xlabel('y')
ylabel('a, b')
legend({'a','b'})
