Selecting and graphing specific frequency range of Spectrogram

Hi! I'm fairly newish to MATLAB and am trying to get the data of a spectrogram to perform an autocorrelation, but only want to do it on a specific frequency range (i.e. from 0 to 50 Hz). How would I get this data and still preserve the time domain?

This is what I've tried, but it only gives me a straight line when I graph f, and I'm pretty sure it's completely wrong.

 figure;
spectrogram(newx, 256, 225, 512, 2000, 'yaxis');
colormap(jet);
[s, f, t] = spectrogram(newx, 256, 225, 512, 2000, 'yaxis');
plot(f);

Thanks in advance for any help!