code: function zoom(ratio, _originalEvent) {
      var canvasData = this.canvasData;
      ratio = Number(ratio);
  
      if (ratio < 0) {
        ratio = 1 / (1 - ratio);
      } else {
        ratio = 1 + ratio;
      }
  
      return this.zoomTo(canvasData.width * ratio / canvasData.naturalWidth, null, _originalEvent);
    }
docs: Zoom the canvas with a relative ratio
  @param {number} ratio - The target ratio.
  @param {Event} _originalEvent - The original event if any.
  @returns {Cropper} this