What's the best way to add a vertical marker to a Pivot Graph to indicate a point in time?

I would like a simple way to add an indicator to a Pivot Graph to mark when something was changed so we can see if a particular change has changed things.

If some text could be add also to indicate what we did, just a few words would be all that is need.

The Pivot Graph will grow with time (the Pivot Table will be updated weekly so the source data in the table will change - the data comes from our server using an sql search within excel) but I obviously would like the marker to stay fixed at that particular date.

Thank you