Last month, I participated in the Open Data Hack Room organized by the University of Berne for their "Night of Research". As a participant, I worked on visualizing some Swiss health data, but that's not what I'm writing about today. Instead, I'd like to go through a very simple example of visualizing some open data, and use it as a starting point to discuss potential sources of error and confusion. It's something I did at around midnight when we were finishing the wine, so I think it's a nice simple starting point: plotting information about referendum votes in the canton of Zurich.