Function fftfreq 

pub fn fftfreq(n: usize, spacing_s: f64) -> Vec<f64>

Frequencies of the DFT bins for a sample spacing spacing_s (numpy.fft.fftfreq): [0, 1, …, ⌈N/2⌉-1, -⌊N/2⌋, …, -1] / (N d).