Hi lepolonais03.
Test case shows that only positive frequencies are considered, as you've obeserved.
sys = zpk(1,[2 3],4);
w=[-100:0.1:-0.1 0.1:0.1:100];
figure
nichols(sys,w)
That is odd, becasue the doc explicitly states that for this syntax:
"The vector w can contain both negative and positive frequencies."
As a workaround, you can use the output form of nichols and make the plot yourself
[mag,phase,wout] = nichols(sys,w);
figure
plot(squeeze(phase),squeeze(db(mag)))
Also, investigate nicholsplot, which might give an option to construct the plot to include negative frequencies.
figure
bode(sys,w)
