Showing the IC50 value on a graph.

Hello. Ive been lurking on here for a while and getting help with excel, now i need some answers!

Im doing a competition assay, and would like to show the IC50 value on the graph. i know how to calculate the IC50, but im not sure how to put it on my graph.

Heres an example of what i would like, a sigmoidal curve and a "box" showing my IC50 value...any suggestions?