any graph can be represented as the set of tuples representing the edges. and in fact this is the most flexible form. it depends on if you need it "human readable" or not, but a hierarchical/network representation of a graph is only one representation

Yeah, it is perfectly possible, I just don't think it is very nice. Neither for parsing nor for human reading.

If I need a file or network message to represent a handful of object types with a bunch of relations that form a graph, I prefer XML (or JSON).