Default filename used by the save method

        Parameters
        ----------
        ext : str
            Extension e.g. png, pdf, ...

def _save_filename(self, ext):
        """
        Default filename used by the save method

        Parameters
        ----------
        ext : str
            Extension e.g. png, pdf, ...
        """
        hash_token = abs(self.__hash__())
        return 'plotnine-save-{}.{}'.format(hash_token, ext)