Extract data from nyquist plot

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!

Answer 1

[Ameer Hamza]

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);

[Andreas Bernatzky]

Thanks Ameer for your solution.

Do you have any clue what goes wrong if you use try to plot like I have done?

[Ameer Hamza]

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]

I see. Thank you!