code: func (d *DFT) ToFreqRange() map[int]float64 {
  	if d == nil {
  		return nil
  	}
  	output := make(map[int]float64, len(d.Coefs)/2)
  	for i := 0; i < len(d.Coefs)/2; i++ {
  		f := (i * d.SampleRate) / (len(d.Coefs))
  		// calculate the magnitude
  		output[f] = math.Log10(math.Sqrt(math.Pow(real(d.Coefs[i]), 2) + math.Pow(imag(d.Coefs[i]), 2)))
  	}
  	return output
  }
docs: // ToFreqRange returns a map with the frequency and their values (normalized)