t = 0 : 5.5*10^-7 : 1*10^-4;
Ym() is a complex array. To plot the magnitudes:
To compute a frequency axis for the plot above:
plot(f,abs(Ym)); xlabel('Frequency (Hz)'); ylabel('abs(Ym)');
Note the symmetry in the plot above. The FFT of a real sequence is conjugate-symmetric about the Nyquist frequency, where fNyquist=Fs/2. Therefore you may limit your plot frequency range to [0,fNyquist] without losing any meaningful information. You may wonder why the FFT is not zero at frequencies other than Fm. The answer is that your sequence does not wrap around smoothly. Also, Fm is not exactly one of the frequencies in the DFT. You can reduce the effect of the wrap-around by windowing the signal.