From the documentation for fftfreq:
>>> signal = np.array([-2, 8, 6, 4, 1, 0, 3, 5], dtype=float)
>>> fourier = np.fft.fft(signal)
>>> n = signal.size
>>> timestep = 0.1
>>> freq = np.fft.fftfreq(n, d=timestep)
>>> freq
array([ 0. , 1.25, 2.5 , 3.75, -5. , -3.75, -2.5 , -1.25])
Why are there negative values in the output array?
I am trying to produce a plot of amplitude vs frequency. I can get the amplitude by running the abs() function over the elements of fourier, but how do I convert freq into a series of frequencies that I can use as an x-axis when plotting fourier?