code: func BGColorFunction(function ColorFunc) OptionsFunc {
  	return func(w *Waveform) error {
  		return w.setBGColorFunction(function)
  	}
  }
docs: // BGColorFunction generates an OptionsFunc which applies the input background
  // ColorFunc to an input Waveform struct.
  //
  // This function is used to apply a variety of color schemes to the background
  // of a waveform image, and is called during each drawing loop of the background
  // image.