Back in the 1980s and early 1990s, the most eye-catching graphic in the Wal-Mart annual report was the “measles map” of the United States, in which each dot represented a company store or distribution center. At first, they were all clustered in Arkansas, Missouri, Oklahoma, and Texas, Wal-Mart’s home turf, but then, year by year, the map charted a seemingly inexorable, disease-like spread as state after state became densely covered with scores of little black dots. The Midwest and South were largely dot-filled by the end of the 1980s; New England and much of the Pacific Coast were spotted less than a decade later. Soon, more than 4,000 dots filled the U.S. map, taking up almost every available space.