private Object readResolve() throws ObjectStreamException {
        if (this.equals(XYCoordinateType.DATA)) {
            return XYCoordinateType.DATA;
        }
        else if (this.equals(XYCoordinateType.RELATIVE)) {
            return XYCoordinateType.RELATIVE;
        }     
        else if (this.equals(XYCoordinateType.INDEX)) {
            return XYCoordinateType.INDEX;
        }     
        return null;