Around 1990 Ben Shneiderman invented TreeMaps as a way to visualize a hierarchy of nodes in a constrained space, for example a rectangle of fixed size. TreeMaps have since been integrated in various tools and are used for interactive graphics in several newspapers and magazines, such as the BBC and NYTimes in the following two examples.
Top 100 Internet sites (Source: BBC online article in Jan 2010)
TreeMap for the TOP 100 Internet Sites as of Jan 2010. In the interactive version (Flash-based) hovering over a rectangle will display the underlying data.
Trucks, Vans, S.U.V. sales in the US (Source: NYTimes article in Feb 2007)
Composite TreeMap of Truck, Van, S.U.V. sales performance in the US by manufacturer..
A history of TreeMaps with many beautiful examples has been compiled by Ben Shneiderman here. My favorite is this variation of a circular TreeMap by Kai Wetzel, a graph showing disk usage by folder with color code showing age of files.
Circular TreeMap showing disk usage (size) and file age (color) of a hierarchical directory structure.
Who says visualizing complex information can’t be informative and aesthetically pleasing at the same time?
visualign 
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