_fillInnerRect(ctx) {
      const { x: outerX, y: outerY } = this._getCoordinates();
      const x = this._caculateInnerPosition(outerX, (outerX[2] - outerX[1]) / 3);
      const y = this._caculateInnerPosition(outerY, (outerY[2] - outerY[1]) / 3);

      ctx.save();
      ctx.strokeStyle = 'rgba(255, 255, 255, 0.7)';
      ctx.lineWidth = this.options.lineWidth;
      ctx.beginPath();

      ctx.moveTo(x[0], y[1]);
      ctx.lineTo(x[3], y[1]);

      ctx.moveTo(x[0], y[2]);
      ctx.lineTo(x[3], y[2]);

      ctx.moveTo(x[1], y[0]);
      ctx.lineTo(x[1], y[3]);

      ctx.moveTo(x[2], y[0]);
      ctx.lineTo(x[2], y[3]);
      ctx.stroke();
      ctx.closePath();

      ctx.restore();
    }
Draw Inner grid line
@param {CanvasRenderingContext2D} ctx - Context
@private