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.

def add_spectrum(self, label, spectrum, color=None):
        """
        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.
        """
        self._spectra[label] = spectrum
        self.colors.append(
            color or
            self.colors_cycle[len(self._spectra) % len(self.colors_cycle)])