class_docstr: Stacked Line graph class
docstr: Add extra values to fill the line
code: def _fill(self, values):
      """Add extra values to fill the line"""
      if not self._previous_line:
          self._previous_line = values
          return super(StackedLine, self)._fill(values)
      new_values = values + list(reversed(self._previous_line))
      self._previous_line = values
      return new_values
  
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 3 for calling the StackedLine obj's underlying member method code with example usage: obj._fill(values) and returns: new_values, super