Treemaps display hierarchical data by using nested rectangles which together represent a whole. They were invented by Ben Shneiderman in 1992 as a way to visualize tree structures in a space-constrained layout.
In a treemap, each branch of a tree is given a rectangle, which is then tiled with smaller rectangles representing sub-branches. The area of each rectangle is proportional to a specified dimension of the data, and the rectangle is often colored to show a separate dimension or to categorize the various rectangles within the treemap. This allows the viewer to easily see patterns that would be hard to spot in bar charts or area charts.
Since treemaps show not only the structure of the tree, but the size of each node, treemaps are particularly useful for demonstrating which parts of the hierarchy have the largest or smallest quantities associated with them.
One of the main benefits of tree maps is that they make efficient use of compact space, so they can legibly display many items on the screen at the same time. That said, treemaps with too many items tend to be hard to read because of the many lines that enclose each small node.
Treemaps are becoming increasing popular, especially for data analysis that requires more detailed views of a large amount of items.