Issues in plotting two graphs in the same plot with hold on function

Hi everybody, i have an issue in plotting the graph with hold on function. Here is the code:

clear all
close all
clc
s = tf('s');

%data
Gp = 4/(s*(-0.4*s-50));
Gs = 5;
Ga = 3;

Kd = 1;
Gf = 1/(Kd*Gs);

% requirement 8
zeta = (abs(log(0.08)))/(sqrt(pi^2+log(0.08)^2))
Tp_max = 1/(2*zeta*sqrt(1-zeta^2))
Sp_max = (2*zeta*sqrt(2+4*zeta^2+2*sqrt(1+8*zeta)))/(sqrt(1+8*zeta^2)+4*zeta^2-1)

% requirement 6
tr_wc = (1/(sqrt(1-zeta^2)))*(pi-acos(zeta))*(sqrt(sqrt(1+4*zeta^4)-2*zeta^2))

% requirement 7
ts_wc = (-log(0.05)/zeta)*(sqrt(sqrt(1+4*zeta^4)-2*zeta^2))

%From the summary
Kc = 100;
nu = 1;

% Design of the controller
Lin = Kc/(s^nu)*Gp*Ga*Gs*Gf

%Bode plot
% figure(2)
% bode(Lin)
% figure(3)
% [num_Lin,den_Lin]=tfdata(Lin,'v');
% nyquist1(num_Lin,den_Lin)

%plot of the forbidden region
figure(4)
myngridst(Tp_max,Sp_max)
zoom on, hold on
nichols(Lin)

I have problems in figure 4 (myngridst is a function provided by the teacher to plot the forbidden region in loop shaping design). In particular, when i run the same code but written few months ago, it works correctly; instead, if i write a new code or i modify the code written some months ago i have these problems:

Instead i should have something like this:

Thank you in advance.

[Cris LaPierre]

As a best practice, always pair a hold on with a corresponding hold off.

We would need to know what your function myngridst is doing. On its own, there is no issue combining a nichols plot with another plot.

plot(linspace(1,20,720))
hold on
H = tf([-4 48 -18 250 600],[1 30 282 525 60]);
nichols(H)
hold off

[Bjorn Gustavsson]

In the screenshot matlab creates a new figure and makes the second plot in that figure, in the second you don't create the second figure and have both plots in the same figure.

[Vinogiga Vanniyakulasingam]

Thank you, i solved the problem.

[Cris LaPierre]

Share your solution as an answer below.

[Bjorn Gustavsson]

@Cris LaPierre: OK

[Cris LaPierre]

Thanks. You can accept your own answer, and by so doing, indicate to others that this question has a solution.

Answer 1

[Bjorn Gustavsson]

In the screenshot matlab creates a new figure and makes the second plot in that figure, in the second you don't create the second figure and have both plots in the same figure.