LayoutAlgorithm computes vertex locations.
LayoutModel accepts a LayoutAlgorithm and contains a Map of vertex to Point locations
VisualizationModel contains a LayoutModel and adds awt classes and event support.
VisualizationServer contains a VisualizationModel and contains affine transforms and renderers to draw a graph.
VisualizationViewer extends VisualizationServer and adds mouse-driven event handling