Networks are often visualized using points and interconnecting lines, with triangular arrowheads at one or both ends to show any directionality between the different points. Although such a standard arrow representation seems intuitive, it can lead to problems in dense graphs that contain many incoming or outgoing relationships. Furthermore, since the arrowheads often have approximately the same size and aspect ratio as the small circles they connect, the graph as a whole might be perceived as cluttered with so much visual detail to the point of being distracting.
Several alternatives of depicting lineair directionality do exist. For instance, lines can vary their hue, luminance or width, along their length. But are these depictions significantly better than just using standard arrows? Which technique performs the best? These questions formed the main focus of the scientific research described in the recent paper "A User Study on Visualizing Directed Edges in Graphs" (PDF) [win.tue.nl] by Danny Holten and Jarke J. van Wijk at the Eindhoven University of Technology.
Their experiments consisted of testing the different visual techniques (or combinations of the techniques), on which participants performed specific tasks in which they had to answer whether or not there were directed connection from one point to another in a randomly generated graph. Response times and accuracy were measured and analyzed. The different techniques tested were: "arrow", "light-to-dark", "dark-to-light", "green-to-red", "curved", and "tapered".
So, you might ask, what can information designers learn from the results from this research?
. A "standard arrow" representation should be avoided whenever possible. Although this representation is straightforward and intuitive to most users, the performance of this popular representation is quite low, which is probably due to the use of arrowheads that cause occlusion problems and visual clutter.
. A "tapered representation" in which the width of an edge is gradually varied along its length - wide at the start and narrow at the end - proved to be the best representation in terms of performance.
. For an "intensity-based" representation (which is still a better choice than the standard arrow representation), a dark-to-light representation has a performance advantage over a light-to-dark representation.
. There does not seem to be a clear performance benefit associated with combining 2 techniques together. Therefore, the researchers recommend to use a single technique instead.
A graphical summary of the relative performance of the different techniques can be found below.