My objection to that map, as someone who was born and raised just north of Dallas, is that no one I've ever met in Dallas County has ever used "coke" to mean carbonated drinks generically without prompting (I will admit some people claim to do so; a claim belied by their use of "soda" in casual speech.)
I lived in Dallas for several years, and this was my observation, too. In some of the outlying counties around Dallas (e.g.-Hunt), "Coke" is definitely prominent, but inside the urban sprawl that is DFW, "soda" is used first and foremost. I think it speaks to just how much of an influx there's been to DFW from the coasts in the past decade or so.
http://i.imgur.com/bGYaa.gif