William S. Cleveland and Robert McGill.
  <b>Graphical Perception: Theory, Experimentation, and
  Application to the Development of Graphical Methods</b>.
  <i>Journal of the American Statistical Association</i>,
  79(387):531-554, September 1984.
  <A HREF="http://vis.cs.brown.edu/docs/pdf/bib/g/Cleveland-1984-GPT.pdf.html">(pdf)</A>
  <A HREF="http://vis.cs.brown.edu/results/bibtex/Cleveland-1984-GPT.bib">(bibtex:
  Cleveland-1984-GPT)</A>.<br />