L1 = tf(3,[1 -1]);

w = logspace(-2,2,1000);
[s_r,s_i] = nyquist(L1,w);

s_r = s_r(:);
s_i = s_i(:);
s_sn = s_r+s_i*i;
s_sp = s_r-s_i*i;

figure(1);
plot(s_sn,'b-','linewidth',2);hold on;
plot(s_sp,'b--','linewidth',2);
axis([-4,1,-2,2]);
grid;

plot(-1,0,'r+','linewidth',2);
xlabel('Re','fontsize',16);
ylabel('Im','fontsize',16);
title('Nyquist Diagram','fontsize',16);
set(gca,'YTick',[-5 0 5]);
set(gca,'XTick',[-5 -1 0 5]);

plot(s_sp(500),'b<','linewidth',6);
plot(s_sn(500),'b>','linewidth',6);

grid on;