Return a |BubbleSeriesData| object newly created and added at the end
        of this sequence, and having series named *name* and values formatted
        with *number_format*.

def add_series(self, name, number_format=None):
        """
        Return a |BubbleSeriesData| object newly created and added at the end
        of this sequence, and having series named *name* and values formatted
        with *number_format*.
        """
        series_data = BubbleSeriesData(self, name, number_format)
        self.append(series_data)
        return series_data