function ObservablePoint(cb, scope, x, y) {
        if (x === void 0) { x = 0; }
        if (y === void 0) { y = 0; }
        this._x = x;
        this._y = y;
        this.cb = cb;
        this.scope = scope;
    }
Creates a new `ObservablePoint`
@param cb - callback function triggered when `x` and/or `y` are changed
@param scope - owner of callback
@param {number} [x=0] - position of the point on the x axis
@param {number} [y=0] - position of the point on the y axis