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Posts: 167
Reply with quote  #1 



I've looked at the treemaps and don't understand why (for this example at least) bar charts wouldn't be (much) preferred.  Figure 6.27 looks ideal.  It certainly would be a lot easier to interpret.  What am I missing?




Bill Droogendyk

Posts: 853
Reply with quote  #2 

I agree that, depending on what you're trying to see in the data, treemaps are often inferior to other media, such as bar graphs. For instance, if you want to compare the quantitative values represented by the sizes or colors of the rectangles in a treemap, you can't do it very well. This is true for two reasons: (1) Our perceptual ability to compare 2-D areas of varying sizes or varying color intensities is very limited and imprecise, and (2) the rectangles are not arranged by size, which makes them difficult to rank and compare.

Ben Shneiderman of the University of Maryland invented treemaps as a means to display two quantitative variables for a large number of items in a small amount of space, arranged hierarchically or categorically (that is, rectangles within larger rectangles to represent the fact that the smaller items belong to the larger groups). They work well primarily for spotting exceptions and overall patterns, such countries with high GDP (rectangle size) and high infant mortality rates (rectangle color) or continent with the least overall level of infant mortality (all countries belonging to the continent have rectangles with low color intensity). I believe that the key to using treemaps successfully is to reserve them for high volumes of data.

For an introduction to treemaps, which you might find informative, keep your eyes open for the April data visualization edition of the Business Intelligence Newsletter from the Business Intelligence Network (www.B-EYE-Network.com), which will be published on Tuesday, April 11, 2006. I've asked Ben Shneiderman to write an article to explain the use of treemaps, which will be featured in the newsletter.

Stephen Few
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