Anscombe's quartet comprises four datasets that have nearly identical simple statistical properties, yet appear very different when graphed.
Each dataset consists of eleven (x,y) points. They were constructed in 1973 by the statistician
Francis Anscombe to demonstrate both the importance of graphing data before analyzing it and the effect of outliers on statistical properties.
It shows how simple statistical measures can fail to show an accurate picture without graphing.
Wikipedia describes this as "All four sets are identical when examined using simple summary statistics, but vary considerably when graphed" -