curveToを使う

/**
 * Copyright _wonder ( http://wonderfl.net/user/_wonder )
 * MIT License ( http://www.opensource.org/licenses/mit-license.php )
 * Downloaded from: http://wonderfl.net/c/9tt0
 */

package {
    import flash.display.Sprite;
    import flash.events.MouseEvent;
    
    public class DrawingCurves extends Sprite {
        private var x0:Number = 100;
        private var y0:Number = 200;
        private var x1:Number;
        private var y1:Number;
        private var x2:Number = 300;
        private var y2:Number = 200;
        
        public function DrawingCurves() {
			init();            
        }
        
        private function init():void {
        		stage.addEventListener(MouseEvent.MOUSE_MOVE, onMouseMove);
        }
        
        private function onMouseMove(e:MouseEvent):void {
        		x1 = mouseX * 2 - (x0 + x2) / 2;
        		y1 = mouseY * 2 - (y0 + y2) / 2;
        		graphics.clear();
        		graphics.lineStyle(1);
        		graphics.moveTo(x0, y0);
        		graphics.curveTo(x1, y1, x2, y2);
        }
    }
}