code: func (w *Waveform) setBGColorFunction(function ColorFunc) error {
  	// Function cannot be nil
  	if function == nil {
  		return errBGColorFunctionNil
  	}
  
  	w.bgColorFn = function
  
  	return nil
  }
docs: // setBGColorFunction directly sets the background ColorFunc member of the
  // receiving Waveform struct.