function plotnyquist(tf,r0,lim);
% Nyquist plot

ax=axes();
theta=angle(tf);
rho=log10(abs(tf)/r0);
plot(rho.*cos(theta),rho.*sin(theta),'-',-log10(1/r0),0,'r.');
text(-log10(1/r0),-0.5,'-1','color','r');
text(rho(1)*cos(theta(1))-0.5,rho(1)*sin(theta(1))-0.5,'\omega = 0','color','b');
text(rho(end)*cos(theta(end))-2,rho(end)*sin(theta(end))+0.5,'\omega = + \infty','color','b');
xlim([-lim,lim]);
ylim([-lim,lim]);
line([0,0],[-lim,lim],'Color','k');
line([-lim,lim],[0,0],'Color','k');
text(0.04*lim,lim-0.06*lim,'Im');
text(lim-0.12*lim,-0.06*lim,'Re');
set(ax,'XTick',[-lim:lim], 'YTick', [-lim:lim]);
set(ax,'XTickLabel',{},'YTickLabel',{});
axis square;