_createLineEventObjectProperties() {
    const params = this.graphics.createObjectProperties(this._line);
    const { x1, x2, y1, y2 } = this._line;

    return extend({}, params, {
      startPosition: {
        x: x1,
        y: y1,
      },
      endPosition: {
        x: x2,
        y: y2,
      },
    });
  }
create line event object properties
@returns {Object} properties line object
@private