Hi Lisa, you cannot have 50 frequencies in 30 elements. Obviously whatever Q is here (is it a power spectral density estimate?) is an estimate at only 30 frequency points.
You can space your frequency vector to cover 5 to 400 Hz with fifty points and then select a subset of those 50 so that you can plot(f,Q)
freq = linspace(5,400,50);
% just making up some junk data to plot
Q = abs(randn(30,1));
plot(freq(1:30),Q)
