Excel milestone/date - marker/indicator

My research has come up empty on trying to add a milestone (date) marker to a very simple graph. Other than drawing the object on the graph itself as I have done does anyone know how to programmatically (via formula/function) or aware of an excel feature which allows you to insert a date (marker or indicator) to a graph? Any assistance would be helpful. I will send a sample of what I have done if it will help come to a resolution.

Thanks!
Mike