I just came across A User Study on Visualizing Directed Edges in Graphs by Danny Holten and Jarke J. van Wijk. The problem they look at in this paper is how to represented directed edges in visualization of graphs, e.g. to represent A is a part of B or A likes B, etc.
Interestingly, their results indicate that what I suspect might be the most commonly used approach (placing an arrow head at the end of the graph) is not the best. Rather, they suggest, using the tapered elements illustrated below is better.
The paper was published at CHI in 2009.
Nice to see these type of applied CHI works :)
Posted by: Siah | April 10, 2011 at 07:59 PM
Without looking at the paper, I can't work out which direction a tapered line is representing. The thinner end could be the direction as it kind of points to something, but the way it fades out confuses me. It looks more like a motion trail to me (picture a commet), where a node is moving away from the other node, which gives it the feeling of having the opposite direction of the first interpretation (in fact, this felt more natural to me).
I'd be interested in hearing other people's thoughts (before you read the paper).
Posted by: Alexbowe | April 11, 2011 at 08:13 AM
@Alexbowe, agreed. They look like tadpoles. Try drawing fancy tapered lines on a whiteboard when you're trying to explain a directed graph to someone. Arrows may be clunky but at least they're simple and unambiguous. Of course, if an artistic interpretation of the data is the goal then do whatever suits your fancy :)
--jacob
@thedatachef
Posted by: TheDataChef | April 11, 2011 at 09:35 AM
I'm agreeing with Alex: I'm not sure what part is what. Suppose it's readership, it makes more sense to have the thinner edge on the writer's side—they tend to have more fans then fans have favorite references. If it's payments though, I'm not sure who send to whom.
Posted by: Bertil_hatt | April 11, 2011 at 10:41 AM
Without reading the paper, I am >90% certain that the line gets fatter in the direction the arrow would point.
Posted by: Robert Renaud | April 11, 2011 at 01:20 PM
Now that I've skimmed the paper, it seems they note the possible ambiguity, but their tests show that people have a faster reaction time.
I guess it might be better if you clearly indicate early on which direction it means.
Posted by: Alexbowe | April 11, 2011 at 11:01 PM