In an example that might be straight out of Tufte, Science News reported recently that Florence Nightingale was an early user of creative information graphics. She used an unusual radial bar chart of hospital deaths during two years of British involvement in the Crimean War, to make the case that better sanitation and better hospital conditions could substantially reduce mortality. The figure in the article is too grainy to read, but an excellent Flash animation makes it much clearer.
When we first examined the graph, some of us made two assumptions: first, that the number of deaths was proportional to the radius of each sector (like a normal rectangular bar chart), and second, that the bars for different causes of death were nonoverlapping (like a stacked bar chart). Both assumptions are wrong: the number of deaths is proportional to the area of each sector, and the sectors do overlap, each one starting from the center of the pie. These assumptions were doubtless driven by familiarity with current bar charts, which probably wasn’t as common for Victorian viewers; nevertheless, it’s interesting to note that our assumptions actually understate the size of the effect. So even to our confused eyes, Florence Nightingale and her graphic designer weren’t lying with this graph; there really are a lot more deaths from preventable disease than from other causes.
One could also ask whether this data would be better displayed as a conventional rectilinear bar chart, with time on the x axis and number of deaths on the y axis. Such a display would be more consistent and more likely to be interpreted correctly, but the radial area chart is certainly more compact, and more interesting too.