I I've been wondering from quite some time now about design issues for argument diagrams. What should a diagram look like to help us make sense? The perceptual features are the reason we put complex issues in diagrams I would argue, but often the diagrams do not naturally give an overview, or show structure. I am not talking about boxes and arrows, but about ways to arrange these boxes, make parts more salient, etc. The argument itself is not changed, but secondary notation (Green & Petre, 1996) helps people in reading/ understanding it, just like titles and white spaces do in text.
I am currently writing a grant on researching design issues in learning with argument diagrams. Would like to discuss these things with others!