A look at national dialects

This map shows generic names for carbonated drinks. Besides being a lovely mosaic of the USA, it tells a story of migration and influence, as well as pockets of disconformity. Why is St. Louis "soda" country at the juncture of "coke" and "pop"? How did Detroit stake out "soda" in the midst of "pop"? What is the "other" used by 50% or more in central New Mexico and in one little district in upstate Michigan?