Mis-read question. Response deleted.
My questions in the forum never get answered!!!
I am sad!!
from what i saw in the api, they are listening to the mouse over on the tip.. they maintain all the tips in the instance cache which is private, so, even if you override the linechart, it is nt possible to listen to the mouseover event on the tip. Wel, I'll do litle more research on this. if you want to do the research yourself, ctrl + click on the datatipfunction you will see a lot of code, find out who is calling it etc.,