How to update Flex Chart every time data is added to its dataProvider?

Now I must be missing something here as it seems like a very simple problem that any Getting Started with Flex tutorial will address. However, all I could find was a hint that the chart should update automatically whenever the DataProvider changes. Keep in mind the hints.

This is my markup:

<?xml version="1.0" encoding="utf-8"?>
<mx:WindowedApplication xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" title="econemon Sensor Simulator">
    <!-- ... -->
    <mx:Script source="SimulatorFunctions.as" />
    <mx:HDividedBox left="0" top="0" bottom="0" right="0">
        <!-- ... -->
        <mx:VDividedBox width="75%" height="100%">
            <mx:PlotChart width="100%" height="33%" dataProvider="{xmlDataTest}">
                <mx:horizontalAxis>
                    <mx:DateTimeAxis dataUnits="seconds" displayLocalTime="true" parseFunction="dtFromUnixtime"/>
                </mx:horizontalAxis>
                <mx:series>
                    <mx:LineSeries xField="dt" yField="fltValue" displayName="Testkurve" />
                </mx:series>
            </mx:PlotChart>
            <mx:Panel width="100%" height="66%">
                <mx:Button label="Start" click="vDataStart()" />
                <mx:Button label="Stop" click="vDataStop()" />
            </mx:Panel>
        </mx:VDividedBox>
    </mx:HDividedBox> 
</mx:WindowedApplication>

      

        
        
        
      

    

And this is part of my ActionScript:

[Bindable]
public var xmlDataTest:Array = [
        { dt: "1230908694", fltValue: "50.4" },
        // ...
        { dt: "1230909594", fltValue: "35.4" }
]

public var dflt:Number = 10.0;
public var timData:Timer = new Timer(3000);

// ...

public function vDataStart():void
{
    if (!timData.running) {
        timData.addEventListener(TimerEvent.TIMER, vAppendDatum);
        timData.start();
    }
}

public function vDataStop():void
{
    if (timData.running) {
        timData.stop();
    }
}

public function vAppendDatum(evt:Event):void
{
    var dtNew:String;
    var fltNew:String;
    var fltT: Number;
    dtNew = String(Number(xmlDataTest[xmlDataTest.length - 1].dt) + 100);
    fltT = Number(xmlDataTest[xmlDataTest.length - 1].fltValue);
    if (fltT < 10.0 || fltT > 70) {
        dflt *= -1;
    }
    fltNew = String(fltT + dflt);
    xmlDataTest.push({ dt: dtNew, fltValue: fltNew });
    //trace("Datenpunkte: " + xmlDataTest.length);
}

      

        
        
        
      

    

When you press the "Start" button, the data array expands every 3 seconds, but the diagram on the screen remains unchanged.