Append a new BubbleDataPoint object having the values *x*, *y*, and
        *size*. The optional *number_format* is used to format the Y value.
        If not provided, the number format is inherited from the series data.

def add_data_point(self, x, y, size, number_format=None):
        """
        Append a new BubbleDataPoint object having the values *x*, *y*, and
        *size*. The optional *number_format* is used to format the Y value.
        If not provided, the number format is inherited from the series data.
        """
        data_point = BubbleDataPoint(self, x, y, size, number_format)
        self.append(data_point)
        return data_point