Analyzing audio by looking at its time-frequency points in real- imaginary plane
This should be a comment but comments have limittaitns

I don't kno if this fixes your problem because I'm not sure what you are doing but using the handel file that comes with matlab

Modifying your code

figure(1);
ii=intersect(find(f&gt;300 ),find(f&lt;500 ));
for j=ii %I just want to look at these frequency indices
plot(real(Sl(:,j)),imag(Sl(:,j)),'Ob'); %grid on;

title(['F=', num2str(f(j)), ' hz']);


produced something.

Hope this helped