By simply adding an "%" sign at the end? You should be much clearer with your questions ( and an example on what you are trying to achieve wouldn't hurt either ).
since ur being so picky about it:
-basically i have a graph and the numbers on the vertical axis range from 1 to 10 and i wanted to create a function that would add a percentage sign after each number
<mx:LinearAxis minimum="0" baseAtZero="false" id="a2"
and you need this function..
private function linearAxis_labelFunc(item:Object, pcat:Object, ax:IAxis):String
var result:String = "";
result = item + "%";