code: def add_series(self, name, number_format=None):
          
          series_data = BubbleSeriesData(self, name, number_format)
          self.append(series_data)
          return series_data
docs: 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*.