Polar = function (name) {

    /**
     * @type {string}
     */
    this.name = name || '';

    /**
     * x of polar center
     * @type {number}
     */
    this.cx = 0;

    /**
     * y of polar center
     * @type {number}
     */
    this.cy = 0;

    /**
     * @type {module:echarts/coord/polar/RadiusAxis}
     * @private
     */
    this._radiusAxis = new RadiusAxis();

    /**
     * @type {module:echarts/coord/polar/AngleAxis}
     * @private
     */
    this._angleAxis = new AngleAxis();

    this._radiusAxis.polar = this._angleAxis.polar = this;
}
Get ring area of cartesian.
Area will have a contain function to determine if a point is in the coordinate system.
@return {Ring}