Simon Rowe answers describes one of my favorite often-inappropriate visuals:
Dr Shneiderman developed the “treemap” in order to visualise this large amount of data—with multiple levels of folders and subfolders—in an efficient way, without taking up too much screen real estate. The treemap uses a series of nested rectangles, sized proportionally to the corresponding data value, to deliver an organised and multi-level view into any hierarchical data set.
Treemaps get misused a lot but are really valuable in specific scenarios. Click through to learn when (and when not) to use a treemap.