[1] Georgia Tech, Graph Visualizat & Usabil Ctr, Atlanta, GA 30332 USA
[2] Georgia Tech, Coll Comp, Atlanta, GA 30332 USA
[3] Univ Kent, Comp Lab, Canterbury CT2 7NF, Kent, England
[4] Univ Lancaster, Dept Comp, Lancaster LA1 4YR, England
[5] Hewlett Packard Labs, Palo Alto, CA USA
[6] AT&T Labs Cambridge, Cambridge CB2 1QA, England