Plot fft

    Args
    ----
    f: ndarray
        Array of frequencies produced with PSD
    S: ndarray
        Array of powers produced with PSD
    dt: ndarray
        Sampling rate of sensor

def plot_fft(f, S, dt):
    '''Plot fft

    Args
    ----
    f: ndarray
        Array of frequencies produced with PSD
    S: ndarray
        Array of powers produced with PSD
    dt: ndarray
        Sampling rate of sensor
    '''
    import numpy

    xf = numpy.linspace(0.0, 1/(2.0*dt), N/2)
    plt.plot(xf, 2.0/N * numpy.abs(S[:N//2]), linewidth=_linewidth)
    plt.show()

    return None