A reader, Rask, just directed me to Anscombe's Quartet on Wikipedia.
It's very important for analysis - it shows how simple statistical measures can fail to show an accurate picture without graphing.
Wikipedia describes this image as "All four sets are identical when examined using simple summary statistics, but vary considerably when graphed" -
Very useful for thinking about. The Wikipedia article goes more into depth, which is important if you're doing numbers-driven analysis. Thanks Rask.