dataTipFunction is called in itemToDataTip() method. You can set function to get datatip for objects or dataTipField and it's call dataTipFunction or just get field of the data.
Of cource you can set dataTipFunction to your DataGrid and save any info about calling this function in your structure, but I cann't understand the reason for this.
If you want to know there is the mouse pointer you can use itemRollOut and itemRollOver events.
You're better off doing this with a function tied to the DataGrid mouseOver or rollOver events.
I'm sorry, I've got
private function datatip(hitData:HitData):String
s = LineSeries(hitData.element).displayName +"\n";
s += LineSeries(hitData.element).yValue;
but I cant seem to get the value to go there. I tried x and y value, I'm sure its simple but I just don't see it.
thanks again, Josh