How to add an average line to a pivot chart

Excellent forum here guys; have learned lots... hoping for some assistance from you experts

I have a pivot table bar chart and trying to a line in this chart of the average of the bar data. I can't find the solution and have search for hours.

- when I select the chart and chose Select Data, everything is grayed out so I can't add a data source. (it's not a read only file)


Thank for any help you can provide!!!