code: def _points(self, x_pos):
          
          for series_group in (self.series, self.secondary_series):
              accumulation = [0] * self._len
              for serie in series_group[::-1 if self.stack_from_top else 1]:
                  accumulation = list(map(sum, zip(accumulation, serie.values)))
                  serie.points = [(x_pos[i], v)
                                  for i, v in enumerate(accumulation)]
                  if serie.points and self.interpolate:
                      serie.interpolated = self._interpolate(x_pos, accumulation)
                  else:
                      serie.interpolated = []
docs: Convert given data values into drawable points (x, y)
          and interpolated points if interpolate option is specified