Petra Neumann Focus+Context Visualization of Relations in Hierarchical Data Otto-von-Guericke-University Magdeburg -UNIVER KE S C IT I Ä R T E U M G A - G N D O E V B - U O R T G T O Faculty of Computer Science Department of Simulation and Graphics Diplom Thesis Focus+Context Visualization of Relations in Hierarchical Data Author: Petra Neumann September 6, 2004 Supervisor: Dr. Stefan Schlechtweg Universität Magdeburg Fakultät für Informatik Postfach 4120, D–39016 Magdeburg Germany Neumann, Petra: Student Number: 157849 Focus+Context Visualization of Relations in Hierarchical Data Diplom Thesis, Otto-von-Guericke University Magdeburg, 2004. 1st Reviewer: Dr. Stefan Schlechtweg 2nd Reviewer: Dr. Sheelagh Carpendale c Petra Neumann Abstract / Zusammenfassung Over the recent years, visualization has become increasingly important in electronic and interactive media. This thesis introduces a new interactive visualization technique tailored to emphasize relations between data elements in a hierarchical data structure. The display of relations between data items in tree structures is commonly solved by grouping items according to color, shape, proximity, size, etc. The approach presented in this thesis, however, represents relations using direct connections. These have been shown to be a stronger grouping principle than proximity, color, size, or shape. The presented visualization technique involves filter and zoom mechanisms to explore the data structures. Automation mechanisms for these operations are introduced through focus+context navigation in the tree. A particular quality of the presented techniques is the consideration of relations in focus+context interaction and presentation which is a novel aspect for tree visualizations. Die visuelle Darstellung von Datensätzen und darin enthaltenen Informationen in in- teraktiven digitalen Medien erhält seit einigen Jahren eine immer größere Bedeutung. Diese Diplomarbeit stellt eine neue interaktive Visualisierungstechnik vor, die in beson- derer Weise die Darstellung von Relationen zwischen Datenelementen in hierarchis- chen Datensätzen ermöglicht. Typischerweise werden Relationen zwischen Datenele- menten in solchen Datensätzen durch Farbcodierung, Verwendung von ähnlichen For- men oder Texturen realisiert. Die Visualisierung, die hier vorgestellt wird, benutzt im Gegensatz dazu eine direkte visuelle Verbindung als Codierungstechnik. Dies basiert auf Untersuchungen, die ergeben haben, dass solche direkten visuellen Verbindun- gen Relationen besser darstellen können als Farbe oder Form. Die vorgestellte Visu- alisierungstechnik beinhaltet außerdem Zoom- und Filtermechanismen um die Daten interaktiv untersuchen zu können. Als ein Automatisierungsmechanismus für diese Interaktionsmöglichkeiten wird Focus+Context Navigation in den Daten vorgestellt. Besonders hervorzuheben ist in diesem Zusammenhang die Beachtung von Relationen in der Focus+Context Navigation und Präsentation, die neuartig für die Visualisierung von Baumstrukturen ist. i Acknowledgement My studies and the work on this thesis would not have been possible and as enjoyable without the support and help of a great many people. First of all I would like to thank my parents and sister for their generous support of my school and university studies in and out of the country. Thanks for all the long trips between Magdeburg and Steinenberg, for helping me move in and out, and for just being a wonderful family. In Magdeburg I would like to thank Stefan Schlechtweg for being an excellent and enthusiastic supervisor. His helpful suggestions gave the work developed here much of its present character. Thanks go also to the professors who made the “Diplomandenlabor” happen that pro- vided me with a great working environment for the completion of this thesis. The interaction and friendship with other students in the lab has contributed greatly to the work presented here. In particular, I am indebted to Angela “Strokewoman” Bren- necke who insisted many times that taking a break is an essential part of the work day and who always shared her great knowledge of LATEX with me. Many thanks also go to my friends Anja, Ingmar, Julia, Laura, Sandra, Uta, René, and all others who made my stay in Magdeburg so much more valuable and enjoyable. My special thanks go to Tobias “Floh” Isenberg who encouraged and supported me since my third semester at university. Thanks for enduring bad-mood times when I had to study for exams, for proof-reading many of my works, in particular this thesis, for introducing me to LATEX, and thanks for being a wonderful friend. I would also like to thank the German Academic Exchange Service (DAAD) for funding my research stays in Canada and Taiwan. iii Contents 1 Introduction 1 1.1 Motivation 1 1.2 Problem Statement 2 1.3 Results 3 1.4 Thesis Organization 3 2 Related Work 5 2.1 Visualization Field 5 2.1.1 Information Visualization 6 2.1.2 Visualization Techniques 7 2.2 Visualization of Linearly Structured Data 12 2.2.1 The Perspective Wall 13 2.2.2 Table Lens 13 2.2.3 Visualizations for Chronological Data 14 2.2.4 Document Visualization 16 2.3 Visualization of Hierarchical Data 18 2.3.1 General tree layout 19 2.3.2 Tree-Maps 21 2.3.3 Information Slices 22 2.3.4 WebTOC 22 2.3.5 Cone Tree and Cam Tree 24 2.3.6 Hyperbolic Browser 25 2.3.7 Cheops 26 2.3.8 Focus+Context Exploration in Hierarchies 26 2.4 Graph Drawing 27 2.5 Visualization of Relations 29 2.5.1 Arc Diagrams 30 v Contents 2.5.2 Thread Arcs 31 2.5.3 Graph Links on Treemaps 31 2.6 Summary 34 3 Visualization of Relations in Hierarchical Data 35 3.1 Problem Analysis 35 3.2 Requirements for a Visualization 36 3.3 Data and Data Representation 38 3.4 Visualization 41 3.4.1 Initial Data Set 41 3.4.2 Spatial Tree Layout 41 3.4.3 Spatial Relation Layout 48 3.4.4 Visualization of Tree Nodes 52 3.4.5 Visualization of Relations 57 3.4.6 Labeling 59 3.5 Summary 60 4 Interaction with a Focus+Context Visualization 63 4.1 Input Device 64 4.2 Gardening Operations 65 4.2.1 Growing and Pruning a Tree 65 4.2.2 Necessary Layout Changes 66 4.2.3 Summary 70 4.3 Focus+Context Navigation 70 4.3.1 Degree of Interest Calculation 71 4.3.2 Lens Interaction 78 4.3.3 Coping with Size 80 4.4 Summary 84 5 Case Studies and Evaluation 85 5.1 Case Studies 85 5.1.1 Structured Documents 86 5.1.2 Sports Tournaments 94 5.1.3 Calendar 98 5.1.4 Study Program Organization 100 5.2 Evaluation 103 5.2.1 Evaluation in HCI 103 5.2.2 Evaluation in Information Visualization 104 vi Contents 5.2.3 Evaluation of the Arc Tree Visualization 105 5.3 Summary 107 6 Conclusion 109 6.1 Summary of Contributions 109 6.2 Future Work 110 6.2.1 Visualization 110 6.2.2 Interaction 111 6.2.3 Evaluation 112 Bibliography 113 List of Figures 121 List of Tables 125 A Appendix 127 A.1 Tree Representations 127 A.2 Graph Theory Definitions 128 A.3 Color Scales 129 A.4 Design Study for the Visualization of Hidden Relations 130 vii CHAPTER 1 Introduction Over the recent years, the combination of visualizations and digital media has become increasingly important. Much of today’s communication involves visualizations in dig- ital form. Aiding users with different tasks by making the underlying data visual is one important research area in computer science. Such research in the area of visualization is largely based on a characteristic of the human visual system: visual information can be processed in parallel and with a high bandwidth into the human cognitive centers (WARE, 2000). This is also the reason why an effective information display can often lead to insight quicker and more memorably than a few pages of written text. This thesis is concerned with the creation of such an effective information display. As one of the most common data types, hierarchical data structures will be visualized with an emphasis on the display of relations between data items. Motivation for the research presented here will be discussed in this chapter. The discussion will give insight into the inherent problems in the development of the visualization and briefly summarize the further development process discussed in subsequent chapters. 1.1 Motivation Knowledge obtained from visualizations often comes from insight gained by recog- nizing that data items are related in some way. In graphs, for example, edges can represent any conceivable kind of relation including those of temporal, causal, or func- tional nature. The display of relations is, therefore, one of the most essential tasks in information visualization. Understanding relations between items in a visualization helps the viewer to build a mental model of the underlying data. This mental model is needed to understand the scheme or situation to which the presented data refers. For making decisions based on the visualization of data the interpretation of this internal 1 Chapter 1 Introduction model is essential. This thesis will introduce a new visualization technique specifically tailored to recognize relations between data elements in a hierarchical data structure. 1.2 Problem Statement In traditional displays of tree structures, edges represent parent-child relations. If more than the inherent parent-child relations needs to be visualized for nodes in a tree, different encoding techniques are necessary. Figure 1.1 affirms this proposition. The figure displays a tree structure in a traditional node-link diagram. Additional links are introduced to represent binary relations between nodes in the tree. From the display parent-child relations and the additional relations cannot be distinguished. In addition, edge-edge intersections are introduced that lead to a cluttered display. Figure 1.1: Additional relations introduced in a traditional tree layout.