Hi,
I know URL is not supported yet. But it's pretty useful already. :)
A note that if you use the scale command, the link in the image map doesn't move. For example, if you render this UML:
scale 200 height
actor "Employee" as EMP
[Tomcat] as APP
[Oracle] as DB
EMP -- APP
APP -- DB
url of APP is [[http://tomcat.apache.org/]]
then the clickable area in the PNG is rendered a little above where the Oracle box is.
Mark