Plot using pydot, graphviz, or GML.

def plt(self, fout_img, engine="pydot"):
        """Plot using pydot, graphviz, or GML."""
        if engine == "pydot":
            self._plt_pydot(fout_img)
        elif engine == "pygraphviz":
            raise Exception("TO BE IMPLEMENTED SOON: ENGINE pygraphvis")
        else:
            raise Exception("UNKNOWN ENGINE({E})".format(E=engine))