code: def appendSubgraph(self, parent_name,  graph_name, graph):
          
          if not self.isMultigraph:
              raise AttributeError("Simple Munin Plugins cannot have subgraphs.")
          if self._graphDict.has_key(parent_name):
              if not self._subgraphDict.has_key(parent_name):
                  self._subgraphDict[parent_name] = {}
                  self._subgraphNames[parent_name] = []
              self._subgraphDict[parent_name][graph_name] = graph
              self._subgraphNames[parent_name].append(graph_name)
          else:
              raise AttributeError("Invalid parent graph name %s used for subgraph %s."
                  % (parent_name,  graph_name))
docs: Utility method to associate Subgraph Instance to Root Graph Instance.
  
          This utility method is for use in constructor of child classes for 
          associating a MuninGraph Subgraph instance with a Root Graph instance.
          
          @param parent_name: Root Graph Name
          @param graph_name:  Subgraph Name
          @param graph:       MuninGraph Instance