Minimizing Visual Clutter in Temporal Treemaps to Enable Comparison of Evolving Hierarchies

Abstract

Evolving hierarchical data describes the temporal development of software repositories, organizational structures, taxonomies in medical science, and merge trees in simulation data, to name a few. Temporal treemaps visualize these evolving trees in a static 2D layout: one dimension shows the hierarchy while the other dimension is devoted to time. Hence, a temporally evolving tree node is displayed as a band of varying width indicating the data value associated with the node. In almost all practically relevant data, these bands meander up and down, vary their width, and cross each other, which is detrimental to the readability of the visualization. Previous methods focused on minimizing the number of crossings, but neglected the meandering: these so-called wiggles are geometric deviations from a straight path. In this paper, we enhance the layout of temporal treemaps by (i) proposing an efficient algorithm for minimizing wiggles in temporal treemaps. Thus, temporal treemaps can be drawn with minimal visual clutter, which is a crucial enabler for (ii) our novel application for temporal treemaps: the interactive visual comparison of two evolving hierarchies. We apply this to tracking algorithms for merge trees, where no such comparative visualizations are available yet. This new application is enabled by (iii) a novel co-optimization of the layout of two temporal treemaps. We show the effectiveness of our approach on previous and new real-world data from a selection of different domains and demonstrate its scalability.