Extract data from nyquist plot

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Hello,
I am trying to extract the data of a nyquist plot and plot it into another figure (I have to do this for another API normally using nyquist would be ok for me).
If i try to plot the data again I get some strange behaviour compared to the original nyquist() plot. Anybody got an idea how to figure this out?
On the left is the nyquist plot created by matlab in
nyquist(G);
and on the right I replot the extracted data
plot(squeeze(re),squeeze(im));
Used lightweight example:
clear all;
close all;
G = tf([1],[1 0.5]);%observerd transfer funciton
G.OutputDelay = 8;
[MAG,PHASE,W] = bode(G);
figure %do "normal" nyquist plot and extract real and imaginary data
[re,im] = nyquist(G);%extract real and imaginary part
nyquist(G);
figure% do own nyquist plot with extracted data
plot(squeeze(re),squeeze(im));
Thanks for you help!

Risposta accettata

Ameer Hamza
Ameer Hamza il 15 Giu 2020
Modificato: Ameer Hamza il 15 Giu 2020
This code shows how you can extract the data from the graphic handles
G = tf(1,[1 0.5]);%observerd transfer funciton
G.OutputDelay = 8;
fig = figure; %do "normal" nyquist plot and extract real and imaginary data
nyquist(G);
hg = findall(fig, 'Type', 'hggroup');
l = hg.Children;
xdata1 = l(1).XData;
ydata1 = l(1).YData;
xdata2 = l(2).XData;
ydata2 = l(2).YData;
figure% do own nyquist plot with extracted data
plot(xdata1, ydata1, xdata2, ydata2);
  3 Commenti
Ameer Hamza
Ameer Hamza il 15 Giu 2020
In your code, you can get a better resolution if you specify 'w' vector yourself
G = tf([1],[1 0.5]);%observerd transfer funciton
G.OutputDelay = 8;
[MAG,PHASE,W] = bode(G);
figure %do "normal" nyquist plot and extract real and imaginary data
w = 0:0.001:10;
[re,im] = nyquist(G, w);%extract real and imaginary part
nyquist(G);
figure% do own nyquist plot with extracted data
plot(squeeze(re),squeeze(im));
Andreas Bernatzky
Andreas Bernatzky il 15 Giu 2020
I see. Thank you!

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