Logichart -- intelligible program diagram for Prolog and its processing system An intelligible program diagram called Logichart has been developed for Prolog visualization. Its syntax rules and layout rules are formalized in terms of an attribute graph grammar. This grammar is underlain by a context-free graph grammar whose productions are defined to formalize the graph-syntax rules of Logichart diagrams. Semantic rules attached to the productions are defined in such a way that they can extract the layout information needed to display a Logichart diagram as attributes. Our Prolog visualization system, which has been implemented based on the attribute graph grammar, can draw a Logichart diagram for any correct Prolog program (completeness), and any Logichart diagram displayed by the system is always valid for the grammar (soundness). The system can also display the execution of a Prolog program in real time; that is, the Logichart diagram is dynamically updated as the program runs