code: def _value_format(self, value, serie, index):
          
          sum_ = serie.points[index][1]
          if serie in self.series and (
                  self.stack_from_top
                  and self.series.index(serie) == self._order - 1
                  or not self.stack_from_top and self.series.index(serie) == 0):
              return super(StackedLine, self)._value_format(value)
          return '%s (+%s)' % (self._y_format(sum_), self._y_format(value))
docs: Display value and cumulation