Danny Holten is an information visualization PhD student within the visualization group of professor Jack van Wijk at the Technische Universiteit Eindhoven - The Netherlands. In the paper "Hierarchical Edge Bundles", which received the best paper award at IEEE InfoVis 2006 for the visualization of adjacency relations in hierarchical data, Holten and his group show a new method for visualizing compound graphs. Their approach is based on visually bundling the adjacency edges, i.e., non-hierarchical edges, together.
As the authors explain: "We assume that the hierarchy is shown via a standard tree visualization method. Next, we bend each adjacency edge, modeled as a B-spline curve, toward the polyline defined by the path via the inclusion edges from one node to another. This hierarchical bundling reduces visual clutter and also visualizes implicit adjacency edges between parent nodes that are the result of explicit adjacency edges between their respective child nodes".
They illustrated the technique by providing examples of visualizations and discussing the results based on an informal evaluation provided by potential users of such visualizations.