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!