Add scale 'sc' and remove any previous
        scales that cover the same aesthetics

def append(self, sc):
        """
        Add scale 'sc' and remove any previous
        scales that cover the same aesthetics
        """
        ae = sc.aesthetics[0]
        cover_ae = self.find(ae)
        if any(cover_ae):
            warn(_TPL_DUPLICATE_SCALE.format(ae), PlotnineWarning)
            idx = cover_ae.index(True)
            self.pop(idx)
        # super() does not work well with reloads
        list.append(self, sc)