Phase diagram from magnitude over frequency (hz)

Hi,

I'm having trouble getting a phase plot out of the my single-sided magnitude spectrum (Hertz) diagram. I have X = Frequency in hertz(1*12800) and Y = magnitude (12800*1) but when I use the plot tool to get the zerophase or the phaseresponse I do not get credible results. I'm new to matlab, any help would be greatly appreciated.

Thank you Tom

[Adam]

You can't get phase information from a magnitude spectrum, you get it from the complex components of the FFT result (assuming your spectrum comes from an FFT, but wherever it comes from it should be complex).

Phase information then comes from using

angle(y);

for some element y