Hierarchical Edge Bundles: Visualization of Adjacency Relations in Hierarchical Data
Compound graphs, a frequently encountered type of data set, have a hierarchical tree structure with parent-child relations (‘inclusion’ relations) and non-hierarchical relations between leaf nodes (‘adjacency’ relations). Such datasets are common in software systems, social networks, and citation networks, amongst other scenarios. Visualizing these data sets is difficult because the addition of adjacency relations in any existing tree visualization method results in visual clutter. Moreover, using a generic visualization approach for these yields poor results too because of the difficulty of separating the inclusion and the adjacency relations. The existing methods for visualizing compound graphs and compound directed graphs (hereafter collectively referred to as compound (di)graphs) have numerous shortcomings, such as the inefficient usage of the available space (radial and balloon layout-based tree visualization techniques), the lack of flexibility (methods for drawing clustered graphs), inability to scale well for compound (di)graphs with large hierarchies (ArcTree-based visualization), unintuitive presentation (matrix view based methods), and excessive clutter because of several “extra routing nodes” introduced by binary splits (flow map layouts). Drawing inspiration from the management and routing of electrical and network cables, this paper presents hierarchical edge bundles for visualizing compound (di)graphs. It is a flexible and an intuitive technique that can be used in conjunction with existing tree visualization methods and reduces visual clutter when working with a high count of adjacency relations. ...