I understand that you want to plot the points (1 + 0j) and (-1 + 0j) to mark them on the Nyquist plot. Consider the following Nyquist plot:
sys = tf(1,[1,1,1]);
nyquist(sys);
The points (1, 0) and (-1, 0) can be marked on the plot as follows:
hold on;
plot(1, 0, "o")
plot(-1, 0, "o")
hold off;