Straight Average Line on Line Chart

Hello All,
This seems like it should be nice and simple yet I (being pretty new to excel) cannot work it out.

I have a list of daily takings. Day to day it varies. I have it on a line chart with markers.

I have added up the takings to find an average.
All I want is to plot that average (if possible as an area chart) overlaying the daily amounts so that I can visually show a better/worse than average day easily.

So one value (that can change dynamically based on future takings) that stretches the whole way across?

I have attached some data.

Thanks again for your assistance