G=tf(1,[conv([1 1],[0.5 1]) 0]);  nichols(G)

rho=[0.1:0.5:5]; G=tf(1,[conv([1 1],[0.5 1]) 0]);
[mm,pp,w]=nichols(G);  ngrid('new')
mm=mm(:); pp=pp(:); 
for i=1:length(rho)
   G.den={[conv([1 1],[0.5 rho(i)]) 0]};
   [m1,p1]=nichols(G,w); 
   mm=[mm m1(:)]; pp=[pp p1(:)];
end
plot(pp,20*log10(mm))

subplot(211), semilogx(w,20*log10(mm))
subplot(212), semilogx(w,pp), 
G_c.num={1}; 
[yy,t]=step(tf(1,[conv([1 1],[0.5 1]) 1]));  
for i=1:length(rho)
   G_c.den={[conv([1 1],[0.5 rho(i)]) 1]};
   y1=step(G_c,t); yy=[yy,y1];
end
figure, plot(t,yy)