code: def add_spectrum(self, label, spectrum, color=None):
          
          self._spectra[label] = spectrum
          self.colors.append(
              color or
              self.colors_cycle[len(self._spectra) % len(self.colors_cycle)])
docs: Adds a Spectrum for plotting.
  
          Args:
              label (str): Label for the Spectrum. Must be unique.
              spectrum: Spectrum object
              color (str): This is passed on to matplotlib. E.g., "k--" indicates
                  a dashed black line. If None, a color will be chosen based on
                  the default color cycle.