Create a Vector for Amplitude Spectrum Plot

Hi, I need to create a program that plots the ampltitude spectrum of a signal with two inputs: Signal and Nyqyust Frequency (phi) and output F which is the FFT of the signal.

How do I create a vector for the x axis which is filled with the frequency values (Hz) which are: 0, 1/N*tau. 2/N*tau, ..., phi, -phi+1/N*tau, -phi+2/N*tau, ..., -1/N*tau ?? Where the first N/2+1 elements of the FFT are positive up to Phi and then it is followed by N/2-1 negative frequencies.

Thanks !