ToolTip Linechart with multiple lines - Header

[LasseLegarth]

Hi,

I want to create a tooltip, that looks like:
Date - Which is my X label
Value : Line1-Name
Value : Line2-name
Value : Line3-name
Value : Line4-name

However, I can't seem to find any option for adding something in the top of the tooltip? only for each line, which isn't what I want.

I can easilly get the date for the BarIndex, but not put it into the tooltip just once. Am i missing something?