These are four x-y data sets, combined into a long format, which have the
same traditional statistical properties (mean, variance, correlation,
regression line, etc.). However, they look quite different.

A data frame with 44 observations on the following 3 variables.

`group`

a categorical identifier for the quartet

`x`

a continuous variable

`y`

a continuous variable

## Details

Data come default in R, but I elected to change the format to be
a bit more accessible.

## References

Anscombe, Francis J. (1973). "Graphs in Statistical Analysis." *The American Statistician* 27: 17–21.