Proportional Circle Map

http://www.google.com/imgres?num=10&hl=en&biw=1680&bih=947&tbm=isch&tbnid=vbuEm_NTl-pcmM:&imgrefurl=http://personal.frostburg.edu/sbriggs0/maps.htm&docid=4YkMQvduLRs2vM&imgurl=http://personal.frostburg.edu/sbriggs0/USA_GradCirclefinal.jpg&w=1650&h=1275&ei=hVTyT97iHob69QT-u6mHAg&zoom=1&iact=rc&dur=438&sig=113213532916660177585&sqi=2&page=1&tbnh=140&tbnw=181&start=0&ndsp=40&ved=1t:429,r:8,s:0,i:98&tx=124&ty=64

This is a proportional circle map, where the circle sizes are proportional to the the data being showing; in this case, Walmart stores in the US in 2009