Question

Using a figure to create a new one, then spectrogram to plot on it, how do I mark a certain point on the existing figure?

Answer 1

Use the hold feature:

hold on

This way, you can make another plot and it will overlay the spectrogram.

An example (you probably didn’t see your marker because it wasn’t large enough):

>> t=0:0.001:2;                    % 2 secs @ 1kHz sample rate  
>> y=chirp(t,100,1,200,'q');       % Start @ 100Hz, cross 200Hz at t=1sec   
>> spectrogram(y,128,120,128,1E3); % Display the spectrogram
>> hold on
>> plot(200,1,'bx','MarkerSize',20,'LineWidth',20); %Overlay a large, blue X