Return an XyDataPoint object newly created with values *x* and *y*,
        and appended to this sequence.

def add_data_point(self, x, y, number_format=None):
        """
        Return an XyDataPoint object newly created with values *x* and *y*,
        and appended to this sequence.
        """
        data_point = XyDataPoint(self, x, y, number_format)
        self.append(data_point)
        return data_point