code: def plt(self, fout_img, engine="pydot"):
          
          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))
docs: Plot using pydot, graphviz, or GML.