"The comparative maps show the number of credit card transactions in Barcelona performed by locals (blue) as well as visitors (pink). It displays data from the week before (left) versus the week during the Mobile World Congress in February/March 2012 (right), in order to reveal the social and economic dynamics of this event in the city."

Source: http://infosthetics.com/archives/2013/02/how_credit_card_usage_changes_during_a_conference.html

Map: http://mwcimpact.com