Plot using the pydot graphics engine.

def wr_pydot_dag(self, fout_img, dag):
        """Plot using the pydot graphics engine."""
        img_fmt = os.path.splitext(fout_img)[1][1:]
        dag.write(fout_img, format=img_fmt)
        self.log.write("  {GO_USR:>3} usr {GO_ALL:>3} GOs  WROTE: {F}\n".format(
            F=fout_img,
            GO_USR=len(self.gosubdag.go_sources),
            GO_ALL=len(dag.obj_dict['nodes'])))