Dot distribution map- The Wal-Mart virus

A dot distribution map shows a distribution of data by placing a dot in the location representing the data.  THis dot distribution map shows the growth of Wal-Mart in the U.S.  It is referred to as the Wal-Mart virus, because of the rapid growth and expansion of Wal-Mart.
http://www.geog.ucsb.edu/~jeff/gis/proportional_symbols.html