Custom renderer for bubble chart

I am trying to create a custom renderer for my bubble chart.
Here is code:
package

import flash.display.GradientType;
import flash.display.Graphics;
import flash.events.MouseEvent;
import flash.geom.Matrix;
import mx.charts.HitData;
import mx.charts.chartClasses.DataTip;
import mx.charts.events.ChartItemEvent;
import mx.containers.VBox;
import mx.controls.Label;
import mx.controls.Text;
public class DataTipRenderer extends DataTip

private var _text:Text = new Text();
private var _vBox:VBox = new VBox();
public function DataTipRenderer()

super();

override protected function updateDisplayList(w:Number, h:Number):void

this.setStyle("textAlign","center");
_vBox.addChild(_text);
this.addChild(_vBox);
_text.text = data.item.ProjectName;

Nothing is coming out.

Thanks.