Visualization of state transition graphs

State transition graphs are important in computer science and engineering where they are used to analyze the behavior of computer-based systems. In such a graph nodes represent states a system can be in. Links, or directed edges, represent transitions between states. Research in visualization investigates the application of interactive computer graphics to understand large and complex data sets. Large state transition graphs fall into this category. They often contain tens of thousands of nodes, or more, and tens to hundreds of thousands of edges. Also, they describe system behavior at a low abstraction level. This hinders analysis and insight. This dissertation presents a number of techniques for the interactive visualization of state transition graphs. Much of the work takes advantage of multivariate data associated with nodes and edges. Using an experimental approach, several new methods were developed in close collaboration with a number of users. The following approaches were pursued: • Selection and projection. This technique provides the user with visual support to select a subset of node attributes. Consequently, the state transition graph is projected to 2D and visualized in a second, correlated visualization. • Attribute-based clustering. By specifying subsets of node attributes and clustering based on these, the user generates simplified abstractions of a state transition graph. Clustering generates hierarchical, relational, and metric data, which are represented in a single visualization. • User-defined diagrams. With this technique the user investigates state transition graphs with custom diagrams. Diagrams are parameterized by linking their graphical properties to the data. Diagrams are integrated in a number of correlated visualizations. • Multiple views on traces. System traces are linear paths in state transition graphs. This technique provides the user with different perspectives on traces. • Querying nodes and edges. Direct manipulation enables the user to interactively inspect and query state transition graphs. In this way relations and patterns can be investigated based on data associated with nodes and edges. This dissertation shows that interactive visualization can play a role during the analysis of state transition graphs. The ability to interrogate visual representations of such graphs allows users to enhance their knowledge of the modeled systems. It is shown how the above techniques enable users to answer questions about their data. A number of case studies, developed in collaboration with system analysts, are presented. Finally, solutions to challenges encountered during the development of the visualization techniques are discussed. Insights generic to the field of visualization are considered and directions for future work are recommended

Visualization of graphs and trees for software analysis

A software architecture is an abstraction of a software system, which is indispensable for many software engineering tasks. Unfortunately, in many cases information pertaining to the software architecture is not available, outdated, or inappropriate for the task at hand. The RECONSTRUCTOR project focuses on software architecture reconstruction, i.e., obtaining architectural information from an existing system. Our research, which is part of RECONSTRUCTOR, focuses on interactive visualization and tries to answer the following question: How can users be enabled to understand the large amounts of information relevant for program understanding using visual representations? To answer this question, we have iteratively developed a number of techniques for visualizing software systems. A large number of these cases consists of hierarchically organized data, combined with adjacency relations. Examples are function calls within a hierarchically organized software system and correspondence relations between two different versions of a hierarchically organized software system. Hierarchical Edge Bundles (HEBs) are used to visualize adjacency relations in hierarchically organized data, such as the aforementioned function calls within a software system. HEBs significantly reduce visual clutter by visually bundling relations together. Massive Sequence Views (MSVs) are used in conjunction with HEBs to enable analysis of sequences of relations, such as function-call traces. HEBs are furthermore used to visually compare hierarchically organized data, e.g., two different versions of a software system. HEBs visually emphasize splits, joins, and relocations of subhierarchies and provide for interactive selection of sets of relations. Since HEBs require a hierarchy to perform the bundling, we present Force-Directed Edge Bundles (FDEBs) as an alternative to visually bundle relations together in the absence of a hierarchical component. FDEBs use a self-organizing approach to bundling in which edges are modeled as flexible springs that can attract each other. As a result, visual clutter is reduced and high-level edge patterns are better visible. Finally, in all these methods, a clear depiction of the direction of edges is important. We have therefore performed a separate study in which we evaluated ten representations (including the standard arrow) for depicting directed edges in a controlled user study

Text in Visualization: Extending the Visualization Design Space

This thesis is a systematic exploration and expansion of the design space of data visualization specifically with regards to text. A critical analysis of text in data visualizations reveals gaps in existing frameworks and the use of text in practice. A cross-disciplinary review across fields such as typography, cartography and technical applications yields typographic techniques to encode data into text and provides the scope for the expanded design space. Mapping new attributes, techniques and considerations back to well understood visualization principles organizes the design space of text in visualization. This design space includes: 1) text as a primary data type literally encoded into alphanumeric glyphs, 2) typographic attributes, such as bold and italic, capable of encoding additional data onto literal text, 3) scope of mark, ranging from individual glyphs, syllables and words; to sentences, paragraphs and documents, and 4) layout of these text elements applicable most known visualization techniques and text specific techniques such as tables. This is the primary contribution of this thesis (Part A and B). Then, this design space is used to facilitate the design, implementation and evaluation of new types of visualization techniques, ranging from enhancements of existing techniques, such as, extending scatterplots and graphs with literal marks, stem & leaf plots with multivariate glyphs and broader scope, and microtext line charts; to new visualization techniques, such as, multivariate typographic thematic maps; text formatted to facilitate skimming; and proportionally encoding quantitative values in running text – all of which are new contributions to the field (Part C). Finally, a broad evaluation across the framework and the sample visualizations with cross-discipline expert critiques and a metrics based approach reveals some concerns and many opportunities pointing towards a breadth of future research work now possible with this new framework. (Part D and E)

Visuelle Suchanfragen auf graphbasierten Datenstrukturen

Die Menge an verfügbaren Daten nimmt stetig zu. Durch standardisierte Datenformate wird die Verknüpfung verschiedener Datenquellen und dadurch auch die Zusammenführung unterschiedlicher Datenelemente je nach Anwendungszweck ermöglicht. Dies führt wiederum zu noch umfassenderen Datenbeständen, in denen die eigentlich gewünschten Informationen teilweise nur schwer gefunden werden können. Handelt es sich bei den Daten um unstrukturierte oder gleichförmige Informationen, so beschränken sich Suchmöglichkeiten auf die Suche nach Übereinstimmungen von Mustern mit Datenelementen oder Teilen davon - beispielsweise Zeichenketten oder regulären Ausdrücken, die mit Teilen von textuellen Datenelementen übereinstimmen. In zunehmendem Maß stehen jedoch auch strukturierte Daten zur Verfügung. Bei diesen wird entweder von Anfang an zwischen unterschiedlichen Facetten pro Datenelement unterschieden, oder es wurden ursprünglich unstrukturierte Daten entsprechend angereichert. Da die einzelnen Facetten auch Verknüpfungen zu anderen Datenelementen darstellen können, entstehen hierbei Graphstrukturen, welche sich für Ansätze der facettierten Suche eignen. Eine Interoperabilität zwischen Datenquellen wird hier unter anderem über die Konzepte und Techniken des Semantic Web erreicht. Zahlreiche Arbeiten haben sich mit der Darstellung der gesamten Datenmengen als Übersicht oder von festgelegten Ausschnitten der Datenmengen im Detail auseinandergesetzt. Jedoch ist das Auffinden bestimmter Daten nach wie vor ein Problem. Die Schwierigkeit liegt dabei darin, die Suchkriterien präzise auszudrücken. Da sich zwischen den einzelnen Kriterien komplexe Zusammenhänge ergeben können, bietet sich auch hier genau wie bei der Übersicht der Datenmengen eine visuelle Darstellung an. Eine Besonderheit dieses Einsatzszenarios für Visualisierungen besteht darin, dass nicht zwangsläufig Daten vorliegen. Statt dessen muss die Visualisierung auch ohne verfügbare Daten die konzeptuelle Idee einer Suchanfrage ausdrücken. Frühere Arbeiten zu diesem Problem befassen sich mit der visuellen Repräsentation von Suchanfragen und Filterausdrücken in Bezug auf relationale Datenbanken und Objektdatenbanken. Viele neuere Arbeiten gehen vermehrt auch auf den Kontext des Semantic Webs ein. Einige dieser Konzepte sind jedoch nicht auf abstrakte Weise klar definiert. Bei komplexeren Anfragen treten zum Teil auch Skalierungsprobleme auf. Zudem wurde bisher kaum betrachtet, wie sich unterschiedliche Konzepte miteinander in Verbindung bringen lassen, um die Vorteile aus unterschiedlichen Anfragevisualisierungen nutzen zu können. Diese Dissertation adressiert die beschriebenen Probleme und stellt sechs Konzepte für die visuelle Darstellung von Suchanfragen vor. Es wird sowohl auf Visualisierungen für allgemeine Einsatzzwecke - also für die Filterung beliebiger strukturierter Informationen -, als auch für spezielle Domänen oder Arten von Informationen eingegangen. Bestehende Ansätze wurden teilweise auf die Gegebenheiten graphbasierter Datenstrukturen angepasst. Ebenso werden neue Ansätze präsentiert, die gezielt auf diese Art von Datenstrukturen ausgelegt sind. Dazu wird jeweils erörtert, inwiefern sich die Anfragevisualisierungen auch ohne Vorhandensein einer zu filternden Datensammlung einsetzen lassen. Zudem wird erklärt, wie bei Vorhandensein einer solchen eine Vorschau auf die Ergebnisse des Filtervorgangs gewährt werden kann. Abschließend werden Verbindungsmöglichkeiten der unterschiedlichen Visualisierungskonzepte präsentiert. Dieser Verbindungsansatz eignet sich dazu, beliebige Anfragevisualisierungen systematisch miteinander zu kombinieren. Mit dem Verbindungskonzept können Benutzer verschiedene Bestandteile einer Anfrage mittels unterschiedlicher Visualisierungskonzepte ausdrücken, um gleichzeitig von den Stärken unterschiedlicher Anfragevisualisierungen zu profitieren. Auf diese Weise können nun Anfragen visuell definiert und dargestellt werden, die sowohl komplexe Bedingungen als auch komplexe Zusammenhänge zwischen den Bedingungen aufweisen, ohne die visuelle Übersicht über einen dieser Aspekte zu verlieren.The total amount of available data is steadily increasing. Standardized data formats allow for connecting different data sources, which can include merging of different data items depending on the use case. This creates even more comprehensive datasets that render finding a particular piece of information difficult. If the data consist of unstructured of homogenous information, searching can only be done by matching patterns with data items or parts thereof - for instance, character strings or regular expressions that match parts of textual data items. However, the availability of structured data is increasing. This kind of data is either stored as distinct facets of each data item from the outset, or originally unstructured data has been enriched to form a structure. As each facet can indicate a link to another data item, the entire dataset forms a graph that is suitable for faceted search conepts. At this point, some interoperability across data sources can be achieved by employing Semantic Web approaches and techniques. Numerous works have attempted to visualize an overview of the entire dataset, or details of a particular excerpt of the dataset. Finding specific data remains a problem, however, as the precise specification of search criteria is difficult. As these criteria can be connected in complex ways, just like the overview of datasets, this issue lends itself to using visual representations. A special trait of this application of visualization is the possible absence of any data. Instead, the visualization must be capable of conveying the conceptual idea of a search query without displaying any data. Former works related to this problem focused on the visual representation of search queries and filter expressions for relational and object-oriented databases. More recent works increasingly address a Semantic Web context. Various of these concepts, however, lack a clear abstract definition. Also, scalability issues appear in the case of complex queries. Furthermore, little attention was paid to how to connect several concepts in order to combine advantages of different query visualizations. This dissertation considers the described problems and presents six concepts for query visualization. Both generic visualizations - that is, for filtering any kind of structured data - and domain-specific or type-specific visualizations are addressed. In part, existing approaches have been adapted to the particularities of graph-based data structures. Likewise, several new approaches specifically designed for this kind of data are presented. For each of these concepts, the necessity of a dataset is discussed. Moreover, options for providing a preview on query results from such a dataset, if available, are considered. Finally, ways for connecting the query visualization concepts are presented. This connection approach is suitable for systematically linking together arbitrary query visualizations. By means of the connection approach, users can express different parts of a query using different visualization concepts, in order to benefit from the advantages of several query visualizations at a time. Like this, queries that include complex criteria as well as complex relations between criteria can now be defined and displayed visually without losing the visual overview of any of these aspects

Graph-level operations: A high-level interface for graph visualization technique specification

More and more the world is being described as graphs---as connections between people, places, and ideas---since they provide a richer model than simply understanding each item in isolation. In order to help analysts understand these graphs, researchers have developed and studied a large number of graph visualization techniques. This variety of techniques presents solutions to a breadth of graph analysis tasks, but it introduces a new issue: complexity. The variety introduces both the complexity of comparing techniques in an objective way and the engineering complexity of implementing so many techniques. In this thesis, I present graph-level operations models (or GLO models) as an elegant solution to these challenges. A GLO model consists of a model of visual elements and a set of functions (GLOs) that manipulate those elements. I introduce GLOv1 and GLOv2, GLO models derived from six hand-picked graph visualization techniques and twenty-nine techniques derived from a review of 430 graph visualization publications, respectively. I show how to use GLOs to define graph visualization techniques, including a model's original seed techniques as well as novel techniques. I demonstrate the analysis potential of the GLO model by clustering the twenty-nine seed techniques using two different GLO-based schemes. Finally, I demonstrate the practical engineering potential of the model through an open-source Javascript implementation (GLO.js) and two applications built atop the implementation for exploring a graph and discovering novel techniques using GLOs (GLO-STIX and GLO-CLI).Ph.D

Cognitive Foundations for Visual Analytics

In this report, we provide an overview of scientific/technical literature on information visualization and VA. Topics discussed include an update and overview of the extensive literature search conducted for this study, the nature and purpose of the field, major research thrusts, and scientific foundations. We review methodologies for evaluating and measuring the impact of VA technologies as well as taxonomies that have been proposed for various purposes to support the VA community. A cognitive science perspective underlies each of these discussions