Exploring Hierarchies Using the DAGMap - Methods for Multilevel Analysis and Visualisation of Geographical Networks - page 104

Fig. 7.6 Use of the gap and cushion to emphasize hierarchical structure. ( a ) Extra space is inserted
between cells to make the nesting structure apparent. ( b ) The cushion effect is computed using a
small shift of cells

Fig. 7.7 Edge bundling

7.2.3 Combined Views

Fekete et al. have extended TreeMaps to general graphs ( Fekete, Wang, Dang, Aris,
& Plaisant , 2003 ) by extracting a spanning tree from the graph and then drawing
edges over the TreeMap. Incidentally, Holten's edge bundling technique ( Holten ,
2006 ) can be used to improve the readability of this technique (Fig. 7.7 ).
We believe our approach is an improvement over previous techniques for
visualizing hierarchical data (DAGs) for two reasons. First, in our case, edges
do not connect two arbitrary cells but correspond to inheritance relationships
that incorporate specific meaning. Users should refer to the Sugiyama layout to
understand inheritance relationships and the level of companies before returning
to the Treemap to view the attributes. Frequently, users switch between these two
views to obtain a complete understanding of the system.