Add missing but required scales.

        Parameters
        ----------
        aesthetics : list | tuple
            Aesthetic names. Typically, ('x', 'y').

def add_missing(self, aesthetics):
        """
        Add missing but required scales.

        Parameters
        ----------
        aesthetics : list | tuple
            Aesthetic names. Typically, ('x', 'y').
        """
        # Keep only aesthetics that don't have scales
        aesthetics = set(aesthetics) - set(self.input())

        for ae in aesthetics:
            scale_name = 'scale_{}_continuous'.format(ae)
            scale_f = Registry[scale_name]
            self.append(scale_f())