Flex中如何通过horizontalStroke样式，自定义LineChart图表水平网格颜色

<?xml version="1.0"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white" viewSourceURL="srcview/index.html">

    <mx:Script>
        <![CDATA[
            import mx.events.ColorPickerEvent;

            private function colorPicker_change(evt:ColorPickerEvent):void {
                var stroke:Stroke = new Stroke(evt.color, 0, 1.0);
                gridLines.setStyle("horizontalStroke", stroke);
            }
        ]]>
    </mx:Script>

    <mx:XMLListCollection id="dp">
        <mx:source>
            <mx:XMLList>
                <quote date="8/27/2007" open="40.38" close="40.81" />
                <quote date="8/24/2007" open="40.5" close="40.41" />
                <quote date="8/23/2007" open="40.82" close="40.6" />
                <quote date="8/22/2007" open="40.4" close="40.77" />
                <quote date="8/21/2007" open="40.41" close="40.13" />
                <quote date="8/20/2007" open="40.55" close="40.74" />
                <quote date="8/17/2007" open="40.18" close="40.32" />
                <quote date="8/16/2007" open="39.83" close="39.96" />
                <quote date="8/15/2007" open="40.22" close="40.18" />
                <quote date="8/14/2007" open="41.01" close="40.41" />
                <quote date="8/13/2007" open="41" close="40.83" />
                <quote date="8/10/2007" open="41.3" close="41.06" />
                <quote date="8/9/2007" open="39.9" close="40.75" />
                <quote date="8/8/2007" open="39.61" close="40.23" />
                <quote date="8/7/2007" open="39.08" close="39.42" />
                <quote date="8/6/2007" open="38.71" close="39.38" />
                <quote date="8/3/2007" open="39.47" close="38.75" />
                <quote date="8/2/2007" open="39.4" close="39.52" />
                <quote date="8/1/2007" open="40.29" close="39.58" />
            </mx:XMLList>
        </mx:source>
    </mx:XMLListCollection>

    <mx:ApplicationControlBar dock="true">
        <mx:Label text="Stroke color:" />
        <mx:ColorPicker id="colorPicker" change="colorPicker_change(event);" />

        <mx:Spacer width="100%" />

        <mx:Legend dataProvider="{lineChart}"
                direction="horizontal" />
    </mx:ApplicationControlBar>

    <mx:LineChart id="lineChart"
            showDataTips="true"
            dataProvider="{dp}"
            width="100%"
            height="100%">

        <mx:backgroundElements>
            <mx:GridLines id="gridLines" direction="horizontal">
                <mx:horizontalStroke>
                    <mx:Stroke color="black" weight="0" alpha="1.0" />
                </mx:horizontalStroke>
            </mx:GridLines>
        </mx:backgroundElements>

        <!-- vertical axis -->
        <mx:verticalAxis>
            <mx:LinearAxis baseAtZero="false" title="Price" />
        </mx:verticalAxis>

        <!-- horizontal axis -->
        <mx:horizontalAxis>
            <mx:CategoryAxis id="ca" categoryField="@date" title="Date" />
        </mx:horizontalAxis>

        <!-- horizontal axis renderer -->
        <mx:horizontalAxisRenderers>
            <mx:AxisRenderer axis="{ca}" canDropLabels="true" />
        </mx:horizontalAxisRenderers>

        <!-- series -->
        <mx:series>
            <mx:LineSeries yField="@open" displayName="Open" />
        </mx:series>
    </mx:LineChart>

</mx:Application>