The clear and meaningful visualization of relations between software entities is an invaluable tool for the comprehension, evaluation, and reengineering of the structure of existing software systems. This paper presents an interaction and representation scheme for the visualization and exploration of complex hierarchical graphs to analyze relations within software systems. Thereby aggregated parts of the software system are represented as treemaps that visualize the structure of the contained software entities. An adaption of existing rectangle-based treemap algorithms for layouts within convex polygonal bounding geometries is introduced to allow for a differentiation of various entity types in the graph visualization. Furthermore, a visual clustering method based on implicit surfaces is presented to create meaningful visualizations of distorted hierarchical graphs of software systems.