Right click chart area

Annotate

Area

Type in "Deficit"

Right click shaded area

Format

Change shading to a colour of your choice (in your case, green)

Pull "Deficit"label outside of shaded area

Shrink/elongate shaded area to a line

Extend "Deficit" label size (label can be resized by pulling the right yellow diamond)

Move both shaded area into position

Move "Deficit" label label into position (label can be moved by grabbing the left yellow diamond)

It may be worth noting, however, that annotations are not dynamic and if the chart changes, the annotation will not update.

You may be better off running a calculated measure that quantifies the difference between two series/measures and then plot it on either a reference line (or a dual axis if you need finer control).