I've never heard Missouri, at least the eastern part around St Louis, called part of the Great Plains. That term generally applies to Kansas, Nebraska, Dakotas, and maybe down to Texas.
Chicago, where you have many posters, isn't on the Great Plains...at least not for the past 150 years!
Here's one map showing what it says are the Great Plains States:
http://en.wikipedia.org/wiki/Great_Plains
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