code: def frequency2fractional(frequency, mean_frequency=-1):
      
      if mean_frequency == -1:
          mu = np.mean(frequency)
      else:
          mu = mean_frequency
      y = [(x-mu)/mu for x in frequency]
      return y
docs: Convert frequency in Hz to fractional frequency
  
      Parameters
      ----------
      frequency: np.array
          Data array of frequency in Hz
      mean_frequency: float
          (optional) The nominal mean frequency, in Hz
          if omitted, defaults to mean frequency=np.mean(frequency)
  
      Returns
      -------
      y:
          Data array of fractional frequency