You want to add Data Labels to your Excel 2013 PowerPivot chart and format as a Percentage.
First right click on your chart bar or line, select “Add Data Labels”, then “Add Data Labels” again.
This will add a data label, but you now need to format it. Right click the actual data label that was just created, and select “Format Data Labels”.
Now under Format Data Labels, scroll down to Number, expand this, set the Category to Percentage and set the number of decimal places.