def add_spectra(self, spectra_dict, key_sort_func=None):
        """
        Add a dictionary of doses, with an optional sorting function for the
        keys.

        Args:
            dos_dict: dict of {label: Dos}
            key_sort_func: function used to sort the dos_dict keys.
        """
        if key_sort_func:
            keys = sorted(spectra_dict.keys(), key=key_sort_func)
        else:
            keys = spectra_dict.keys()
        for label in keys:
            self.add_spectra(label, spectra_dict[label])