Burkay Genc Ugur Dogrusöz A Constrained, Force-Directed Layout Algorithm for Biological Pathways. 314-319 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2912&spage=314 2003 conf/gd/2003 Graph Drawing db/conf/gd/gd2003.html#GencD03