How come the graph edges are "--" and the digraph edges are "->", but not "<-"? That means it's not possible to flip graph/digraph to see how it looks with/wit

How come the graph edges are "--" and the digraph edges are "->", but not "<-"? That means it's not possible to flip graph/digraph to see how it looks with/without arrows, but naievely it doesn't seem that the difference makes any difference except making that difficult, if the connections must always be written left to right. (I've not used more advanced features, so it could be that it does more, or could be that it was built expecting people to know what they want first, which would all be fine)