Updated: January 15, 2008; G. Waloszek
Treemap is a hierarchy-viewing object that displays up to thousands of elements at once showing multiple attributes. By recursively partitioning an element's rectangle among its children, a treemap presents the "big picture" of a hierarchy giving users X-ray vision into their mountains of data. Zooming, enter/leave notification, select actions, and much more are supported.
From HCIL Website: Treemap is a space-constrained visualization of hierarchical structures. It is very effective in showing attributes of leaf nodes using size and color coding. Treemap enables users to compare nodes and sub-trees even at varying depth in the tree, and help them spot patterns and exceptions. Treemap was first designed by Ben Shneiderman during the 1990s.
Ben Shneiderman, B. Johnson (HCIL); 1991