It's not so much poor food selection as poor access to quality and affordable food. You can eat for a lot longer on $10 if you're buying things on the McDonald's dollar menu than if you're buying fresh produce at Safeway or giant.
Meanwhile, my local grocery store has 2lbs of chicken thighs for $5. You can add some potatoes and onions for another couple bucks. So for roughly the same price a single person could get 2-4 meals. The difference is that for the second meal the person needs time to prepare.
Potatoes often cost less than $0.50 a pound.
I think those are very valid comparisons, potatoes are even more nutritious than anything McDonald's sells.
Sure, you have to cook them both for 10 or 15 minutes, which means you have to have a working stove top, but McDonald's is not more affordable than the grocery store, especially if you are buying things that are only comparably nutritious (rather than for flavor or better nutrition or whatever).
Here, the decent groceries are a bit over a mile away, two are a bit closer than the McDonald's, one is a bit further. 2 Subways beat them all.
I guess such a map would have uninteresting regions where it was more of a choice (rural residential lots).