class_docstr: Stacked Line graph class
docstr: Display value and cumulation
code: def _value_format(self, value, serie, index):
      """
      Display value and cumulation
      """
      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))
  
file_path: pygal/pygal/graph/stackedline.py
imports: from pygal.adapters import none_to_zero
  from pygal.graph.line import Line
label_desc: Use image node_id 2 for calling the StackedLine obj's underlying member method code with example usage: obj._value_format(value, serie, index) and returns: unknown, super