Tim, I have done that… What I found is that when I plot by cases per capita, the top few zip codes are so much larger than the rest that it renders my whole colormapping scheme ineffective… i.e., I have two zip codes that are bright red, one that is light blue, and the rest are all dark blue, regardless of their numbers. The border zip codes are the ones with the really high per capita numbers… one zip near Yuma is getting close to 100%, no kidding! Interestingly, I found that if I pick the highest 20% of the zip codes by per capita cases, they ALSO represent 80% of all the cases! Lots we could discuss there over steaks! 🙂 Also, I don’t have numbers by Zip for most of Navajo and Apache due to tribal regulations. That’s why those two counties are a big blank…

Wouldn’t be better if you had a similar map based on per capita numbers rather than nominal. Santa Cruz/Apache/Navajo counties have high per capita numbers but don’t make your map due to low gross numbers. Then attack the zip codes with the highest percentage of penetration rather than just the populated zip codes.