Return an |XySeriesData| object newly created and added at the end of
        this sequence, identified by *name* and values formatted with
        *number_format*.

def add_series(self, name, number_format=None):
        """
        Return an |XySeriesData| object newly created and added at the end of
        this sequence, identified by *name* and values formatted with
        *number_format*.
        """
        series_data = XySeriesData(self, name, number_format)
        self.append(series_data)
        return series_data