-val gDot: ?packing:(GObj.widget -> unit) -> unit -> graphviz_widget
-val gNeato: ?packing:(GObj.widget -> unit) -> unit -> graphviz_widget
-val gTwopi: ?packing:(GObj.widget -> unit) -> unit -> graphviz_widget
-val gCirco: ?packing:(GObj.widget -> unit) -> unit -> graphviz_widget
-val gFdp: ?packing:(GObj.widget -> unit) -> unit -> graphviz_widget