Formalising pragmatic features of graph-based notations

Graph-based notations form a significant subclass of visual languages. Studies of the use of such notations in practice have shown that users often employ pragmatic aspects, such as layout, to capture important domain information. Moreover this pragmatic information can support and guide reasoning over such representations. However, typical formalisations of graph-based notations often pay scant regard to such pragmatic considerations. This paper highlights an algebraic account of graph-based notations which is sensitive to relevant layout information. We illustrate, with examples taken from software engineering practice, how this algebra both captures pragmatic aspects of graphs and supports direct reasoning over their structure.