Adds a dos for plotting.

        Args:
            label:
                label for the DOS. Must be unique.
            dos:
                PhononDos object

def add_dos(self, label, dos):
        """
        Adds a dos for plotting.

        Args:
            label:
                label for the DOS. Must be unique.
            dos:
                PhononDos object
        """

        densities = dos.get_smeared_densities(self.sigma) if self.sigma \
            else dos.densities
        self._doses[label] = {'frequencies': dos.frequencies, 'densities': densities}