Kenneth M. Goldberg and Boris Iglewicz.
Bivariate Extensions of the Boxplot.
In Technometrics, vol. 34, no. 3, pp. 307--320, 1992.

Bibtex:

@Article{        goldberg:1992:BEBP,
  author = 	 "Kenneth M. Goldberg and Boris Iglewicz",
  title = 	 "Bivariate Extensions of the Boxplot",
  journal = 	 "Technometrics",
  year = 	 "1992",
  volume = 	 "34",
  number = 	 "3",
  pages = 	 "307--320",
  month = 	 "August",
}