Impressively, the chart was created with just 14 lines of R code:
(To recreate it yourself, download the GEOSTAT-grid-POP-1K-2011-V2-0-1.zip file from eurostat, and move the two
.csv files inside in range of your R script.) The code parses the latitude/longitude of population centers listed in the CSV file, arranges them into a 0.01 by 0.01 degree grid, and plots each row as a horizontal line with population as the vertical axis. Grid cells with zero populations cause breaks in the line and leave white gaps in the map. It's quite an elegant effect!
Update: James Cheshire created a version of this concept back in 2013 (again using R of course). You can see (and purchase a print) of a map of the world with population lines, which was used for the Royal Geographical Society’s Annual Conference in 2013.