in Sections 4.2 and 4.3. An asset of physical modeling that is often overlooked is its inherent exibility. For this reason, we conclude this chapter by listing examples of model speci cations tailored to speci c layout objectives. 4.1 The Springs Given a connected undirected graph with no particular background information, the following two criteria of readable layout seem to be generally agreed upon for the conventional two-dimensional straight-line representation. 1. Vertices should spread well on the page. 2. Adjacent vertices should be close. Only intuitive explanations can be oered. While uniform vertex distribution reduces clutter, the implied uniform edge lengths leave an undistorted impression of the graph. Since \clutter" and \distortion" already have physical connotations, it seems fairly natural to start thinking of a more speci c physical analogy. We are used to observing even spacing between repelling objects. This makes it natural to imagine vertices behaving l

An orthogonal drawing of a graph is an embedding of the graph in the rectangular grid, with vertices represented by axis-aligned boxes, and edges represented by paths in the grid which only possibly intersect at common endpoints. In this paper, we study three-dimensional orthogonal drawings and provide lower bounds for three scenarios: (1) drawings where vertices have bounded aspect ratio, (2) drawings where the surface of vertices is proportional to their degree, and (3) drawings without any such restrictions. Then we show that these lower bounds are asymptotically optimal, by providing constructions that match the lower bounds in all scenarios within an order of magnitude.

In this paper we investigate the general position model for the drawing of arbitrary degree graphs in the D-dimensional (D >= 2) orthogonal grid. In this model no two vertices lie in the same grid hyperplane.

with respect to edges can be of interest as well. A method to do this can be found in Paulish (1993, Chapter 5). Clustering of graphs means grouping of vertices into components called clusters. Thus, clustering is related to partitioning the vertex set. Denition 8.1 (Partition). A (k-way) partition of a set C is a family of subsets (C 1 ; : : : ; C k ) with { S k i=1 C i = C and { C i \ C j = ; for i 6= j. The C i are called parts. We refer to a 2-way partition as a bipartition. Now, we can dene one of the most basic denitions of clustered graphs. 8. Drawing Clusters and Hierarchies 195<F14.

This paper presents the first non-trivial lower bounds for the total number of bends in 3-D orthogonal graph drawings with vertices represented by points. In particular, we prove lower bounds for the number of bends in 3-D orthogonal drawings of complete simple graphs and multigraphs, which are tight in most cases. These result are used as the basis for the construction of infinite classes of c-connected simple graphs, multigraphs, and pseudographs (2 &le; c &le; 6) of maximum degree &Delta; (3 &le; &Delta; &le; 6), with lower bounds on the total number of bends for all members of the class. We also present lower bounds for the number of bends in general position 3-D orthogonal graph drawings. These results have significant ramifications for the `2-bends problem', which is one of the most important open problems in the field.

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