Thanks a bunch, that's exactly what I needed, here's how the code ended up.
Mark <mx:verticalAxisRenderer> <mx:AxisRenderer> <mx:labelRenderer> <mx:Component> <mx:Label toolTip="{this.myTip}" > <mx:Script> <![CDATA[ [Bindable] private var myTip:String; override public function set data(value:Object):void{ if(value == null) return; myTip = value.text; var length:int = value.text.toString().length; if (length > 20){ text = value.text.toString().substr(0, 20) + "..."; } else { text = value.text; } } ]]> </mx:Script> </mx:Label> </mx:Component> </mx:labelRenderer> </mx:AxisRenderer> </mx:verticalAxisRenderer>