gantt tooltip formatting + extra html

Hi,

I'm trying to add some extra information to the GANTT visual tooltip. Best would be, if there's any way to use formatted HTML/CSS. Does anybody know

1. if it is possible without changing code?
2. where to start off inside the code of GANTT visual to add such functionality

I had a look at the code but could figure out, where the actual rendering is done (just file name and line number would help a lot).

Any help is appreciated, thanks.

Carsten