this.startDrag (false, _x , 0, _x + _width, 0); will
constrain to the x and y axis, (copied from my current project,
changes the 0's to move along y). In order to do what you want you
need to uuse he onMouseDown function along with onEnterFrame (if
you want a 'real-time' update ofthe x-coordinate, or simply
onMouseUp. You have to use onMouse up either way.