Plot using pydot, graphviz, or GML.

def plt_dag(self, fout_img, engine="pydot"):
        """Plot using pydot, graphviz, or GML."""
        if engine == "pydot":
            self._plt_pydot(fout_img)
        else:
            raise RuntimeError("ENGINE NOT IMPLEMENTED({E})".format(E=engine))