Flex: LineChart提示：不是编译时常数

看到一个Flex的例子，程序代码如下：但是遇到错误，哪位帮忙看看怎么回事
Error: 找不到类型，或者它不是编译时常数: LineChart

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="vertical"> 
<mx:XML source="t.xml" id="stockData" /> 
<mx:Panel title="Stock Data" width="100%" height="100%" layout="vertical" paddingBottom="10" paddingLeft="10" paddingRight="10" paddingTop="10">
<mx:HBox>
<mx:Label text="Start Day" />
<mx:HSlider minimum="0" maximum="30" id="dayslider" snapInterval="1" />
</mx:HBox>
<mx:LineChart id="chart" dataProvider="{stockData..day.(num >= daySlider.value )}" width="100%" height="100%">
<mx:series>
<mx:LineSeries xField="num" yField="compa" displayName="Company A" />
<mx:LineSeries xField="num" yField="compb" displayName="Company B" />
</mx:series>
</mx:LineChart>
<mx:Legend dataProvider="{chart}" />

</mx:Panel>
</mx:Application>

t.xml的内容如下：

 <days>
 <day>
 <num>0</num>
 <compa>966.429108587301</compa>
 <compb>920.7133933216961</compb>
 </day>
 <day>
 <num>1</num>
 <compa>956.429108587301</compa>
 <compb>520.7133933216961</compb>
 </day>
 </days>

Error: 找不到类型，或者它不是编译时常数: LineChart