Given a DAG, return a pydot digraph object.

def _get_pydot_graph(self):
        """Given a DAG, return a pydot digraph object."""
        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