During the Discovery 2008 conference, I heard a number of requests for tornado charts. Later, I realized a tornado chart is basically a horizontal, stacked bar chart with one negative variable and one positive variable, which is something JMP 7 can already handle. They're also called back-to-back charts, comparative histograms and population pyramids. Here's an example showing US population by age group:
Steps for creating this kind of plot in JMP:
Make sure there are two X variables (age and sex) and one Y variable (population). Tables Summary or Stack may be required to get the data in this form.
Make every other Y value negative (the females in the example).
Launch Chart with the two Xs and one Y and Horizontal orientation turned on.
Turn on Stack Bars within Chart.
Hide the divider lines by setting their color to off-white in Customize in the right-click menu.
Use the Grabber (hand) tool to move the legend into the graph.
So now that we can make tornado charts, what good are they? What kinds of data are they best for? How is the above tornado chart better or worse than the normal side-by-side bar chart (below)? Tell us what you think.