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GRAPH DRAWING and INFORMATION VISUALIZATION conduconduit t! Volume 8, Number 1 Department of Computer Science Spring, 1999 Brown University Traditionally, information visualization GRAPH DRAWING and has focused on the display of quantita- tive information (e.g., bar charts, pie INFORMATION VISUALIZATION charts, function plots) and geographic information (e.g., road and subway Information visualization is maps), where a natural mapping exists an emerging discipline that between the data and their geometric addresses the problem of location in the diagram. A great introduc- communicating the struc- tion to these types of visualizations is ture of information space given in the books by Edward Tufte (The through diagrams. Quot- Visual Display of Quantitative Informa- ing from an article by David tion and Envisioning Information). Harel (Comm. ACM 31(5) 1988): From quantitative to relational “The intricate nature of More recently, researchers have started a variety of computer- addressing the problem of visualizing related systems and sit- relational information, where net- uations can, and in our works (also known as graphs) model opinion should, be rep- collections of objects and connections resented via visual for- between those objects. Examples include: malisms; visual because • Web: site maps, browsing history dia- they are to be gener- grams, presentation and refinement ated, comprehended, of query results, product catalogs. Roberto Tamassia and communicated by humans; and attending a workshop formal, because they are to be manip- • Software engineering: UML class and state-transition diagrams, sub- on graph drawing in ulated, maintained, and analyzed by Barbados computers.” “...in the next decade the use The benefits of informa- tion visualization include of information visualization analysis through visual exploration, discovery of techniques will be essential to patterns and correlations, the success of portals and other and abstraction and sum- marization. It is antici- large information repositories pated that in the next decade the use of informa- on the Internet” tion visualization techniques will be routine-call graphs, data-flow dia- essential to the success of portals and grams. other large information repositories on • Database systems: entity-relation- the Internet. ship diagrams. Brown University, Box 1910, Providence, RI 02912, USA • Real-time systems: Petri nets and To complicate matters, some basic graph state-transition diagrams. drawing problems for which theoretically • Networking: LAN diagrams. fast algorithms are known turn out to be • Enterprise and project manage- unwieldy to implement. Take, for in- ment: business process diagrams, stance, the problem of testing whether a organization charts, scheduling graph is planar, i.e., whether it can be charts. drawn without crossings. While mathe- • Engineering: circuit schematics. matical characterizations of planar • Artificial intelligence: knowledge graphs have been known since the 18th representation diagrams, belief and century, it was only in 1974 that John influence networks. Hopcroft and Robert Tarjan published the first linear-time algorithm to test The fundamental problem in the visual- whether a graph is planar (J. ACM ization of relational information is the 21(4)). This algorithm was a major theo- automatic layout of networks, which is retical accomplishment and greatly con- the subject of the research area known as tributed to their earning the prestigious graph drawing. Several pioneering Turing Award. commercial applications have begun to appear. For example, the AltaVista Coming up with a correct implementa- search engine by DEC/COMPAQ can tion of the algorithm was, however, a dif- visualize the results of a query with an ferent matter. The intrinsic conceptual automatically generated drawing of a difficulty of the approach combined with graph whose vertices are relevant key- data-structuring tricks and special cases words, and supports the refinement of defeated the efforts of many program- the query by a direct manipulation of the mers for more than 20 years until finally drawing. The Hyperbolic Tree (TM) tech- in 1996 a research team led by Kurt nology for drawing trees by Inxight Soft- Mehlhorn completed the development of ware (part of the Xerox New Enterprise a reliable implementation of the business initiative) is used in the Web Hopcroft-Tarjan planarity testing algo- rithm that has been successfully tested on tens of thousands of graphs. Graph drawing research at Brown Since my first paper on automatic layout of entity-relationship diagrams was pub- lished in 1983, graph drawing has been one of my main research interests. Three of my six doctoral students to date have done their research on graph drawing: AltaVista Web site of the Wall Street Journal (go to search display the “Money Tree”) and is incorporated in a Web management tool by Microsoft. There is significant potential for enhanc- ing electronic commerce Web sites with graph-drawing technology. Bob Cohen Ashim Garg Robert Cohen (Ph.D. 1992, now at Algo- While the task of automatically produc- magic Technologies, Inc.), Ashim Garg ing a readable layout for a graph may (Ph.D. 1995, now at SUNY Buffalo), and appear simple to a nonexpert, it is actu- Stina Bridgeman (current; a piece on her ally computationally very hard. The cost research appears in this issue). of incorporating effective automatic net- work-layout capabilities into software My recent work on graph drawing has systems is often grossly underestimated. focused on: conduit! 2 • interactive layout techniques includes the development and commer- • Web-based graph-drawing systems cialization of a package of Java software • software design patterns for graph components for graph layout in collabora- drawing tion with Algomagic Technologies, a recent startup founded by Robert Cohen, Graph drawing on the Web Michael Goodrich from Johns Hopkins The Graph Drawing Server is a Web- University and myself. based system that provides graph draw- ing services. It can be accessed in vari- A book on graph drawing ous ways: I have recently published a book on the • through an interactive graph editor subject in collaboration with three other implemented as a Java applet graph-drawing gurus: Giuseppe Di Bat- tista from the University of Rome, Italy, • through an HTML form that allows Peter Eades from the University of New- the definition of the input graph in a castle, Australia, and Ioannis Tollis from variety of formats the University of Texas at Dallas. The rig- • through a Java package that pro- orous treatment of the subject allows this vides an API for involving the server book to be used as a text for graduate It supports various layout styles and drawing algorithms, including orthogo- nal layouts computed by the GIOTTO algorithm and hierarchical layouts Graph-drawing gurus. l to r: Ioannis Tollis, Roberto, Giuseppe Di Battista and Peter Eades courses and as a reference for research- ers, while the large number of examples and figures makes it also suitable for soft- Graph drawing server ware practitioners. The latest issue of Dr. Dobb’s Journal computed by an algorithm that distrib- (June 1999, page 134) mentions the book utes vertices on horizontal layers, and as follows: has been successfully used by many researchers worldwide to experiment “The final book this month is with graph-layout techniques. GRAPH DRAWING: Algorithms for the Visualization of Graphs,by Graph drawing in Java Giuseppe Di Batista, Peter Eades, Roberto Tamassia, Ioannis G. Tollis. I am developing a library of reusable The title is an accurate summary of software components for graph drawing the book’s contents, but doesn’t do in Java. This work is based on algorithm justice to its breadth. Section 5.1, for engineering techniques that include the example, is devoted to angles in use of novel algorithmic patterns. The orthogonal drawings, while chapter library can be used to incorporate auto- 7 covers incremental construction matic layout capability in various inter- technniques. The style is academic— faces that make use of diagrams. I plan there are a lot of references, and a to show its applications especially to Web lot of proofs and lemmas—but the browsers and programming environ- book will be a rich mine of ideas for ments. anyone who is trying to persuade a In addition to algorithmic and systems computer to turn data into dots, research, my future graph-drawing work boxes, lines and arrows.” conduit! 3 “HORIZON” This imposing iron and azure glass monument by artist Costas Varotsos was commissioned by General and Mrs. Kanellakis in memory of their son Paris and his family. With majes- tic Mt. Parnassos as a back- drop and the town of Liya below, the sculpture is located on family-owned land—a favorite summer haunt where they’d gather each year to take family snapshots. The inscription reads: DEDICATED TO OUR CHILDREN PARIS - MATE - ALEXANDRA - STEPHANOS THEIR PARENTS ELEFTHERIOS AND ROULA KANELLAKIS 20-12-95 Paris’s legacy will endure via fellowships and awards established to honor his memory—the ACM’s Kanellakis Award, Brown’s Kanellakis Graduate Fellowship and MIT’s Kanellakis Graduate Fellowship, to name a few. His parents have donated the land around the monu- ment, aptly, to the SOS Children’s Village International Parc. Said his mother, “It was their favorite summer place on earth and now their spirits shall dwell there—near to us—while those we cherish are young and together in eternity.” using the technologies effectively and/or THE UNIQUENESS OF CS92 on a large scale. He writes
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