Adding markers on specific points in bodeplot - matlab

Question

I am currently designing a 5th order Butterworth filter and looking at its transfer function response in Matlab. I have successfully calculated it and have plotted its bode response like this:
% Butterworth Fifth Order Low Pass
figure(1)
h = bodeplot(FinalTF);
setoptions(h,'FreqUnits','Hz','PhaseVisible','off');
title('Butterworth LowPass Fifth Order');
grid on;
where FinalTF is the transfer function I'm talking about. What I want is to add markers on specific points in this plot (specifically I want to highlight the frequencies fp,fo,fs, you don't need to know what these are, they're just 3 different points on the x-axis, and the dB at each frequency) with code. I know how to do it by clicking on the graph, but that will be too time consuming, as I have many plots to go through. I am currently running into two basic problems:
1) I don't know how to get the specific dB at each frequency just by using the TF object. I tried using the function evalfr(), but tbh the values it returns seem a bit off.
2) Ignoring the previous point, even if I do the calculations by hand, I can't add them on the plot using this method, and I'm not sure what the problem is. Maybe because I'm using bodeplot instead of regular plot? I don't know how else to do it though.
I'm using Matlab 2015, if it makes any difference.
Any help would be appreciated. Thanks in advance.

Answer 1

I actually found a solution. Here is some sample code to illustrate the results. Before I was doing:
figure(3);
h = bodeplot(FinalTF);
setoptions(h,'FreqUnits','Hz','PhaseVisible','off');
grid on;
title('Chebyshev 2nd Order Band Pass');
It just printed the bodeplot of the transfer function. Now I managed to add markers to the specific frequencies I wanted like this:
figure(4);
myW = logspace(1,5,1000);
myW = 2*pi*myW;
[mag,~,wout] = bode(FinalTF,myW);
mag = squeeze(mag);
wout = squeeze(wout);
mag = 20*log10(mag);
wout = wout/2/pi;
semilogx(wout,mag,'-b');
axis([min(wout) max(wout) min(mag)-10 max(mag)+10]);
title('Chebyshev 2nd Order Band Pass');
xlabel('Frequency (Hz)');
ylabel('Magnitude (dB)');
grid on;
hold on;
freqMarkers = [w0 w1 w2 w3 w4];
[dbMarks,~,freqMarks] = bode(FinalTF,freqMarkers);
dbMarks = squeeze(dbMarks);
freqMarks = squeeze(freqMarks);
dbMarks = 20*log10(dbMarks);
freqMarks = freqMarks/2/pi;
semilogx(freqMarks,dbMarks,'r*');
And works great! Thanks to #Erik for the help.

Answer 2

You can use [mag,~,wout] = bode(sys) and then plot(wout,mag) to create the Bode plot. Then, using hold on and plot(...), you can add whatever points you need to the plot.
Note that wout is in radians per TimeUnit, which is a property of sys (source). To convert wout to a frequency axis in Hertz, you can set TimeUnit in sys using sys.TimeUnit = 'seconds' (which is the default, so probably unnecessary) and then f = wout/2/pi;. Plot it using plot(f,mag), then hold on and plot your markers.
To calculate the magnitude at certain frequencies, use mag = bode(sys,w); where w are the frequencies in radians per sys.TimeUnit. If sys.TimeUnit is 'seconds' and you frequencies are in Hertz, use w = 2*pi*f, where f are the frequencies you need.