Only Godless heathens call soda, "pop".
St. Louis is over the top about calling it soda. Places will act like they have no idea what you want when you order "pop." They know, they are just jerks.
That map is like bright red over St. Louis...little "soda" capital of the Midwest.
I say soda for the most part, but I ALWAYS say "pop" in St. Louis. eff those assholes, man.