The TreeMap is a constant area visualisation invented by Ben Shneiderman for hierarchical data with some form of numerical attribute, for example the sizes of files and folders on a computer disk. The most basic form starts with a rectangular area and then recursively divides the area alternatively horizontally using the total values for each level.
Links:
cs.umd.edu: Treemaps for space-constrained visualization of hierarchies- Wikipedia: Treemapping