When did Brooklyn declare independence?

I know that Brooklyn likes to differentiate itself from NYC, and Manhattan in particular, usually by having its denizens wear t-shirts that say "Brooklyn" on them. But I'm pretty sure it is still part of NYC. That makes this map by Mint.com showing the most frugal cities in the US confusing. The whole map is confusing for that matter. This is a nice example of the wrong way to visualize data.