The plot is based on information derived from R's "datasets" package - US State Facts and Figures.

As per the R Documentation:

Source

U.S. Department of Commerce, Bureau of the Census (1977) Statistical Abstract of the United States. U.S. Department of Commerce, Bureau of the Census (1977) County and City Data Book.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.