Beyond Clustered Planar Graphs

Many real-world networks exhibit an inherent clustering structure, which may arise from the presence of communities inside the network or from semantic affinities among nodes. Constructing effective visualizations for such networks is a crucial task that poses several practical and theoretical challenges. The standard theoretical model for readable representations of clustered graphs is the one, of c-planarity, introduced in the 90s and still a central research topic in graph drawing. The goal of this model is to realize drawings avoiding unnecessary crossings involving edges or clusters. This chapter reviews alternative models that have been proposed to enlarge the set of clustered graphs allowing for a representation that still conveys the clustering and relational information. First, we deal with a relaxed notion of c-planarity, in which some crossings are allowed. Then, we present two popular models for hybrid representations of clustered networks, namely NodeTrix and Intersection-Link representations, which combine different drawing paradigms for the inter-cluster and the intra-cluster relationships.