If there's an unwritten law of digital map making it is this: given a data set with a geographical element, someone, somewhere, will probably make a map out of it.
A prime example of this law is mining data from Encyclopedia Metallum detailing the Heavy Metal bands per country and mashing it up with population data from the CIA World Factbook (yes, this really exists) to make a map of Heavy Metal bands, by country, per capita.
While I'm not the biggest fan of heavy rock, the resulting map does, err, rock.
Credit is also due to my lovely wife who, knowing my penchant for all things map related, pointed this out to me in the first place.
Image Credits: Reddit user depo_s via GIS Lounge.