Anscombe's quartet

"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." (Wikipedia)

Anscombe, F. J. (1973). "Graphs in Statistical Analysis". American Statistician 27 (1): 17–21