AS3 Draggable layer/mask on y axis

programming flash
1

I’d like to create this exact example: http://www.nytimes.com/2008/12/28/nyregion/18thennow.html?_r=2& however have the drag occur on the y axis instead of the x. I was working with some basic code, but wondering how to lock it in place?

myImage.mask = myMask;
stage.addEventListener(MouseEvent.MOUSE_DOWN, fMouseDown);
stage.addEventListener(MouseEvent.CLICK, fMouseUp);
function fMouseDown(e:MouseEvent):void {
myMask.startDrag();
}
function fMouseUp(e:MouseEvent):void {
myMask.stopDrag();
}

It isn’t quite what I need though, any help? Thanks in advance!