T. M. J. Fruchterman and E. M. Reingold.
  <b>Graph drawing by force-directed placement</b>.
  <i>Softw. Pract. Exper.</i>, 1991.
  <A HREF="http://vis.cs.brown.edu/docs/pdf/bib/g/Fruchterman-1991-GDF.pdf.html">(pdf)</A>
  <A HREF="http://vis.cs.brown.edu/results/bibtex/Fruchterman-1991-GDF.bib">(bibtex: Fruchterman-1991-GDF)</A>.<br />