William S. Cleveland and Robert McGill.
  <b>Graphical Perception and Graphical Methods for Analyzing
  Scientific Data</b>.
  <i>Science</i>, 229:828-833, 1985.
  <A HREF="http://vis.cs.brown.edu/results/bibtex/Cleveland-1985-GPG.bib">(bibtex:
  Cleveland-1985-GPG)</A>.<br />