BubbleChart is not using the label function
===

I have created a BubbleChart and I've set the label function as shown below. However, it's not getting called. 


	    <mx:BubbleChart width="100%" height="100%"
						minRadius="3" maxRadius="16"
						color="#ffffff"
						dataProvider="{dataSet}">
            <mx:series>
                <mx:BubbleSeries radiusField="P" 
								 showDataEffect="{eff}" 
								 xField="A" 
								 yField="B">
					
					<mx:verticalAxis>
						<mx:LinearAxis padding="5" 
									   labelFunction="axisLabelFunction"
								
									   />
					</mx:verticalAxis>
					<mx:horizontalAxis>
						<mx:LinearAxis padding="5" />
					</mx:horizontalAxis>
				</mx:BubbleSeries>
            </mx:series>
        </mx:BubbleChart>

And the function call:  

			public function axisLabelFunction(labelValue:Object, previousValue:Object, axis:IAxis):String {
				
				if (labelValue) { // breakpoint here - never triggered
					trace("value");
				}
				return "";
			}