adding a line to CartesianDataCanvas

Is it possible? I need to draw a line on the background.

My code:
<mx:annotationElements>
<mx:CartesianDataCanvas id="canvas" includeInRanges="true">
</mx:CartesianDataCanvas>
</mx:annotationElements>

var _line:UIComponent = new UIComponent();
_line.graphics.lineStyle(5);
_line.graphics.moveTo(0, 60);
_line.graphics.lineTo(100, 60);
canvas.addChild(_line);

No error, no line.

Thanks