Hi,
I understand that you are trying to create a grey highlighting strip between 10 Hz and 13 Hz on the x-axis of “pwelch” plot. It is not required to find out the corresponding y-axis indices and the limits before creating a patch since an entire grey column from top to bottom is required. Instead, you can simply set the y-limits. I have added a sample example below to show the same.
clear;
fs = 1000;
t = 0:1/fs:5-1/fs;
x = cos(2*pi*100*t) + randn(size(t));
[powspect,hzp] = pwelch(x,500,300,500,fs);
lowerlim=200;
higherlim=250;
pwelch(x,500,300,500,fs);
%%shade region in plot
hold on;
ylim([-50,40])
ph= patch([lowerlim higherlim higherlim lowerlim], [ [1 1]*min(ylim) [1 1]*max(ylim)],[ 0.5 0.5 0.5],'FaceAlpha',0.3, 'EdgeColor','none');
hold off;
In this example I have used dummy data to create “pwelch” plot. You can modify the “ylim,”” lowerlim” and “higherlim” based on your use case.
You can refer to the following documentation link for more information on the “pwelch” function:
