The challenge is how to reduce noise when constructing meta- (or cluster?) difographs, either statically or (I hope) dynamically. That is, how best to reduce the number of top-level map-to-map connections.

I suggest the map "portal", an element created at the time the map is first created. Creation of a map would thus create two elements -- the one containing the map title and the second, two-way linked to it, the map portal. Data contained in the link between the two would include the number of maps linking to the portal (i.e. to this map/difograph).

All links from other maps to the map in question would go to this portal and not to the top-level map title element. The link would be a two-way link and information about the link would be coded into it. 

The portal would be a color and shape-coded in order to differentiate it from other elements, in the same way the rectangular yellow nodes indicate links to other maps.