Transform values in a dataframe.

        Returns dataframe

def transform_df(self, df):
        """
        Transform values in a dataframe.

        Returns dataframe
        """
        if (len(df) == 0) or (len(self) == 0):
            return df

        # Each scale transforms the columns it understands
        for sc in self:
            df = sc.transform_df(df)
        return df