ABSTRACT
In many cases a directed graph represents a hierarchy and we want to draw it in this way. We will define a hierarchy later, but for now it is sufficient to think of a hierarchy as a cyclefree digraph where it is useful for nodes of the graph to be stratified into discrete, parallel layers. Examples of hierarchies or near-hierarchies are, among others, PERT charts for project management, object-oriented class diagrams, and function call graphs from software engineering. As usual, nodes represent entities and edges represent relationships between the entities. Closely related to hierarchically layered drawings and discussed later are radial drawings where nodes are placed on concentric circles [DDLM04, Bac07] and cyclic level drawings, where nodes are placed on “spokes” emanating from a centre-point [BBBF12].
