flex 下图形问题（急等）

小弟最近在做一个flex项目，需要做一个竞争力排名的图形，需要Y轴式负坐标，也就是y轴在x轴下面的那种，但所有有的例子都是Y轴为正坐标的，想请问各位是需要对哪个属性进行调整吗

---

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
layout="vertical"
verticalAlign="top"
horizontalAlign="center"
backgroundGradientColors="[0x000000,0x323232]"
paddingTop="0"
viewSourceURL="srcview/index.html">

<mx:Script>
<![CDATA[
import mx.collections.ArrayCollection;

[Bindable]
private var medalsAC:ArrayCollection=new ArrayCollection([{Country: "USA", Gold: -35, Silver: 39, Bronze: -29}, {Country: "China", Gold: -32, Silver: -17, Bronze: 14}, {Country: "Russia", Gold: -27, Silver: 27, Bronze: -38}]);
]]>
</mx:Script>

<mx:Panel title="ColumnChart Control"
layout="horizontal"
color="0xffffff"
borderAlpha="0.15"
width="600"
height="240"
paddingTop="10"
paddingRight="5"
paddingBottom="10"
paddingLeft="5"
horizontalAlign="center">

<mx:ColumnChart id="