FFT Plot Axis Confirmation
Here's a short snippet of code that returns the domain of the FFT depending on if you have the onesided or twosided FFT. This is from some code I posted on a somewhat related question here.

% Nfft - FFT size
% Fs - Sampling frequency in Hz
% oneside - flag indicating FFT results will be onesided or two sided

% Compute the frequency vector
if oneside == true
  f = fs * (0:Nfft/2).'./Nfft;
else
  f = fs * (-Nfft/2:Nfft/2-1).'./Nfft;
end