plotting nullclines in matlab

beta = 0.0004 ;
k = 4000 ;
delta = 0.0225;
S = [0: 150];
I1 = (1-(S/k))/(1/k + beta);
plot(S,I1)
hold on;
y = ylim ;
plot([50 50],[y(1),y(2)],'-.g')
xlabel('S');
ylabel('I');
title('Phase Plane Plot');
plot(S(51),I1(51),'r*')
legend('S Nullclines','I Nullclines','Equilibrium Point');
hold on;

i was wondering if this code made sense

[David Goodmanson]

Hi Alex,

could you post the original equations that this is based on?