Diagram Layout - reduce crossing nodes, too?
 

Using v4.1.2 Pro (recently upgraded from v2!), I am building a series of concept maps (different from mind maps in small but important ways). So far, I've been letting OmniGraffle lay out the map, which works well in general, but as the maps get more complex (i.e. connections spanning multiple levels in the outline hierarchy, etc), lines are regularly crossing concepts (boxes). There's a "reduce crossings" option in the diagram layout inspector, but that seems to only affect the likelihood of a connection crossing other connections.

So, is there any way to hint that connections shouldn't *ever* cross concepts, or (more reasonably to me) shouldn't all automatic layout modes avoid allowing connections to cross concepts, regardless of the hint setting?