Frequency Axis problem in a DTFT
Generally I like to compose my sinusoids using this format:

x = cos(2*pi*f/fs*(0:num_samps-1))


Depending on the FFT routine you will use it will provide either the onesided or twosided result. Calculating the domain of the frequency axis is as follows. It should be noted that num_samps and NFFT may not be the same.

% 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


Have a look at this paper for a nice explanation on spectrum estimation.