code: def _get_pydot_graph(self):
          
          rel = "is_a"
          pydot = self._get_pydot()
          # Initialize empty dag
          dag = pydot.Dot(label=self.title, graph_type='digraph', dpi="{}".format(self.dpi))
          # Initialize nodes
          go2node = self._get_go2pydotnode()
          # Add nodes to graph
          for node in go2node.values():
              dag.add_node(node)
          # Add edges to graph
          rel2col = self.pltvars.rel2col
          for src, tgt in self.godag.get_edges():
              dag.add_edge(pydot.Edge(
                  go2node[tgt], go2node[src],
                  shape="normal",
                  color=rel2col[rel],
                  dir="back")) # invert arrow direction for obo dag convention
          return dag
docs: Given a DAG, return a pydot digraph object.