With my understanding on your query ,
1. If you want Label and Data as pop up ,
First make sure that you have choosen Combo Chart (Combination of Bar and Line graph) as your Chart Type.. Then just like adding an expression , add your AVG(Expression) on to the expressions list and on the Display Options check on Line and Text as Pop up box ..
If you want to show the avg trend line to something like avg cost, then name your expression to avg cost and please check your Legend settings ...This will show you both the Label and data whenever you mouse over on the average line you will get the data pop up (avg value)...
2. If you want to show the label all the time,
Checkout on the rightside of presentation tab where you can add Text or expression, as Dennis said..
Hope this will help you !!!