def node_name
      path = self.class.to_s
      if i = path.rindex('::')
        path = path[(i+2)..-1]
      end
      path[0] = path[0].chr.downcase
      path
    end
The node name to use in serialization. As AreaChart is used as the
base class for Liine3DChart we need to be sure to serialize the
chart based on the actual class type and not a fixed node name.
@return [String]