Seok-Hee Hong

Effects of Crossing Angles

In visualizing graphs as node-link diagrams, it is commonly accepted and employed as a general rule that the number of link crossings should be minimized whenever possible. However, little attention has been paid to how to handle the remaining crossings in the visualization. The study presented in this paper examines the effects of crossing angles on performance of path tracing tasks. It was found that the effect varied with the size of crossing angles. In particular, task response time decreased as the crossing angle increased. However, the rate of the decrease tended to level off when the angle was close to 90 degrees. One of the implications of this study in graph visualization is that just minimizing the crossing number is not sufficient to reduce the negative impact to the minimum. The angles of remaining crossings should be maximized as well.

Measuring effectiveness of graph visualizations: a cognitive load perspective

Graph visualizations are typically evaluated by comparing their differences in effectiveness, measured by task performance such as response time and accuracy. Such performance-based measures have proved to be useful in their own right. There are some situations, however, where the performance measures alone may not be sensitive enough to detect differences. This limitation can be seen from the fact that the graph viewer may achieve the same level of performance by devoting different amounts of cognitive effort. In addition, it is not often that individual performance measures are consistently in favor of a particular visualization. This makes design and evaluation difficult in choosing one visualization over another. In an attempt to overcome the above-mentioned limitations, we measure the effectiveness of graph visualizations from a cognitive load perspective. Human memory as an information processing system and recent results from cognitive load research are reviewed first. The construct of cognitive load in the context of graph visualization is proposed and discussed. A model of user task performance, mental effort and cognitive load is proposed thereafter to further reveal the interacting relations between these three concepts. A cognitive load measure called mental effort is introduced and this measure is further combined with traditional performance measures into a single multi-dimensional measure called visualization efficiency. The proposed model and measurements are tested in a user study for validity. Implications of the cognitive load considerations in graph visualization are discussed.

Fáry’s Theorem for 1-Planar Graphs

A plane graph is a graph embedded in a plane without edge crossings. Fary’s theorem states that every plane graph can be drawn as a straight-line drawing, preserving the embedding of the plane graph. In this paper, we extend Fary’s theorem to a class of non-planar graphs. More specifically, we study the problem of drawing 1-plane graphs with straight-line edges. A 1-plane graph is a graph embedded in a plane with at most one crossing per edge. We give a characterisation of those 1-plane graphs that admit a straight-line drawing. The proof of the characterisation consists of a linear time testing algorithm and a drawing algorithm. Further, we show that there are 1-plane graphs for which every straight-line drawing has exponential area. To the best of our knowledge, this is the first result to extend Fary’s theorem to non-planar graphs.

GEOMI: GEOmetry for maximum insight

This paper describes the GEOMI system, a visual analysis tool for the visualisation and analysis of large and complex networks. GEOMI provides a collection of network analysis methods, graph layout algorithms and several graph navigation and interaction methods. GEOMI is part of a new generation of visual analysis tools combining graph visualisation techniques with network analysis methods. GEOMI is available from http://www.cs.usyd.edu.au/~visual/valacon/geomi/.

A graph reading behavior: Geodesic-path tendency

The end result of graph visualization is that people read the graph and understand the data. To make this effective, it is essential to construct visualizations based on how people read graphs. Despite the popularity and importance of graph usage in a variety of application domains, little is known about how people read graphs. The lack of this knowledge has severely limited the effectiveness of graph visualizations. In attempts to understand how people read graphs, we previously observed that people have geodesic-path tendency based on subjective eye tracking data. This paper presents two controlled experiments. One is to approve the existence of the geodesic-path tendency. The other is to examine the effects of this tendency on people in reading graphs. The results show that in performing path search tasks, when eyes encounter a node that has more than one link, links that go toward the target node are more likely to be searched first. The results also indicate that when graphs are drawn with branch links on the path leading away from the target node, graph reading performance can be significantly improved.

Order-preserving matching

We introduce a new string matching problem called order-preserving matching on numeric strings, where a pattern matches a text if the text contains a substring of values whose relative orders coincide with those of the pattern. Order-preserving matching is applicable to many scenarios such as stock price analysis and musical melody matching in which the order relations should be matched instead of the strings themselves. Solving order-preserving matching is closely related to the representation of order relations of a numeric string. We define the prefix representation and the nearest neighbor representation of the pattern, both of which lead to efficient algorithms for order-preserving matching. We present efficient algorithms for single and multiple pattern cases. For the single pattern case, we give an O(nlogm) time algorithm and optimize it further to obtain O(n+mlogm) time. For the multiple pattern case, we give an O(nlogm) time algorithm.

Automatic visualisation of metro maps

We investigate the new problem of automatic metro map layout. In general, a metro map consists of a set of lines which have intersections or overlaps. We define a set of aesthetic criteria for good metro map layouts and present a method to produce such layouts automatically. Our method uses a variation of the spring algorithm with a suitable preprocessing step. The experimental results with real world data sets show that our method produces good metro map layouts quickly.

Improving multiple aesthetics produces better graph drawings

Many automatic graph drawing algorithms implement only one or two aesthetic criteria since most aesthetics conflict with each other. Empirical research has shown that although those algorithms are based on different aesthetics, drawings produced by them have comparable effectiveness.The comparable effectiveness raises a question about the necessity of choosing one algorithm against another for drawing graphs when human performance is a main concern. In this paper, we argue that effectiveness can be improved when algorithms are designed by making compromises between aesthetics, rather than trying to satisfy one or two of them to the fullest. We therefore introduce a new algorithm: BIGANGLE. This algorithm produces drawings with multiple aesthetics being improved at the same time, compared to a classical spring algorithm. A user study comparing these two algorithms indicates that BIGANGLE induces a significantly better task performance and a lower cognitive load, therefore resulting in better graph drawings in terms of human cognitive efficiency.Our study indicates that aesthetics should not be considered separately. Improving multiple aesthetics at the same time, even to small extents, will have a better chance to make resultant drawings more effective. Although this finding is based on a study of algorithms, it also applies in general graph visualization and evaluation. Highlights? We have introduced cosine force, which is to increase the size of crossing angles. ? We have introduced sine force, which is to increase the angular resolution of vertices. ? We have introduced and implemented a force-directed algorithm, which improves multiple aesthetics at the same time. ? We have conducted a human study, which was the first to examine the collective effect of multiple aesthetics on humans. ? We have presented empirical evidence showing that improving multiple aesthetics produces better graph drawings.

Beyond time and error: a cognitive approach to the evaluation of graph drawings

Time and error are commonly used to measure the effectiveness of graph drawings. However, such measures are limited in providing more fundamental knowledge that is useful for general visualization design. We therefore apply a cognitive approach in evaluations. This approach evaluates graph drawings from a cognitive perspective, measuring more than just time and error. Three user studies are conducted to demonstrate the usefulness of this approach.

Effects of sociogram drawing conventions and edge crossings in social network visualization

This paper describes a user study examining the eects of dierent spatial layouts on human sociogram perception. The study compares the relative eectiveness of ve sociogram drawing conventions in communicating the underlying network substance, based on task performance and user preference. The impact of edge crossings is also explored by using social network specic tasks. Both quantitative and qualitative methods are employed in the study.