The fine people over at Sports Charts have provided a map of where NBA players come from, and L.A. has produced the most with 16, beating out New York which had 14, and Chicago with 13. There aren't too many NBA players period, so it's not surprising that the three biggest cities have the most players. What is a bit surprising is that this map uses just the city data, and New York City has almost 2x as many people as the city of Los Angeles. If the map were expanded to use metro areas, it's possible L.A. would have even more players.
