Edward Tufte has a lovely page dedicated to the slopegraph, a simple and elegant visualization tool.

The Iowa caucuses are rapidly approaching, the first contest in a lengthy presidential preference primary season.

Ann Selzer is an extraordinarily talented pollster in Iowa. She regularly polls Iowa, and her final polls are extremely accurate.

For the last few presidential caucus cycles, she has polled every several weeks leading up to the caucus. I thought I would offer visualizations of recent contests of her polling in her penultimate poll, usually about 5 to 8 weeks out from the caucus, and the actual outcome. Polling data from 2004, 2008, and 2012 have been rounded, as have outcomes.

(The polling outcomes do not necessarily correlate with the actual convention delegates awarded, but they simply reflect the percentage of votes each candidate received. The Democratic caucuses include special rules that permit several rounds of voting, and caucusgoers can change their support after each round. I exercised discretion in the order I listed candidates tied. I only included candidates who received some modest showing of support in either the penultimate polling or the caucus results.)

UPDATE: Someone helpfully altered me on Twitter that another poll was conduct in 2012 in late November. So the chart above reflects the antepenultimate poll rather than the penultimate poll.