A large class of diagrams can be informally characterized as node–link diagrams. Typically nodes represent entities, and links represent relationships between them. The discipline of graph drawing is concerned with methods for drawing abstract versions of such diagrams. At the foundation of the discipline are a set of graph aesthetics (rules for graph layout) that, it is assumed, will produce graphs that can be clearly understood. Examples of aesthetics include minimizing edge crossings and minimizing the sum of the lengths of the edges. However, with a few notable exceptions, these aesthetics are taken as axiomatic, and have not been empirically tested. We argue that human pattern perception can tell us much that is relevant to the study of graph aesthetics including providing a more detailed understanding of aesthetics and suggesting new ones. In particular, we find the importance of good continuity (ie keeping multi-edge paths as straight as possible) has been neglected. We introduce a methodology for evaluating the cognitive cost of graph aesthetics and we apply it to the task of finding the shortest paths in spring layout graphs. The results suggest that after the length of the path the two most important factors are continuity and edge crossings, and we provide cognitive cost estimates for these parameters. Another important factor is the number of branches emanating from nodes on the path.

The UML syntactic notation used in texts, papers, documentation and CASE tools is often different, despite UML being considered a software engineering standard. Our initial empirical study considered variations in the notation used for UML class diagrams; the experiment reported here concentrates on UML collaboration diagrams.The decision as to which of the semantically equivalent notational variations within the UML standard to use appears to be according to the personal preference of the author or publisher, rather than based on any consideration of the ease with which the notation can be understood by human readers. This paper reports on an experiment that takes a human comprehension perspective on UML collaboration diagrams. Five notations were considered: for each, two semantically equivalent (yet syntactically or stylistically different), variations were chosen from published texts. Our experiment required subjects to indicate whether a supplied pseudo-code specification matched each of a set of experimental UML collaboration diagrams.The results reveal that our informal, personal intuitions (which were based on our view of the complexity of the notation) are validated with respect to confirming that a specification matches a diagram, but not when errors in a diagram are to be identified. The subjects' preferences are in favour of the more concise notational variants.

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