Saturday, June 20, 2009
A graphical bullseye
In the latest issue of American Scientist magazine there is an article, That’s Funny, that discusses two examples which show how what we emphasize can make it either easy or hard to find data in a graphic. A complicated example uses the famous bullseye chart shown here. It very concisely depicts which of 3 antibiotics are best targeted to control growth of bacteria in glass lab culture dishes (in vitro).
Three columns in the table at the right also show results for those 3 antibiotics: penicillin, streptomycin, and neomycin. Each entry in the table shows the concentration of antibiotic required to stop bacteria from growing (the minimum inhibitory concentration, acronymed as MIC). Values for the MIC cover a huge range: from 0.001 to 850. The fourth column in the table shows whether the bacteria are colored by Gram stain when viewed with a microscope.
The chart shows how Will Burtin plotted the data in 1951 on a logarithmic circular chart, like a rifle target with a bullseye. The outermost ring is an MIC of 0.001, followed by 0.01, 0.1, 1.0, 10, 100, and 1000. In the bullseye chart the response to Gram stain is color coded as purple for positive and orange for negative.
The article points out how other graphics are required to show some puzzling behavior exhibited by a few of those types of bacteria.