code: func (d *DFT) BinWidth() int {
  	if d == nil {
  		return 0
  	}
  	if d._binWidth > 0 {
  		return d._binWidth
  	}
  	d._binWidth = (d.SampleRate / 2) / len(d.Coefs)
  	return d._binWidth
  }
docs: // BinWidth is the width of a bin (in frequency).
  // It is calculate by using the Nyquist frequency (sample rate/2) divided by
  // the DFT size.