docs: function mxCellOverlay(image, tooltip, align, verticalAlign, offset, cursor)
  {
  	this.image = image;
  	this.tooltip = tooltip;
  	this.align = (align != null) ? align : this.align;
  	this.verticalAlign = (verticalAlign != null) ? verticalAlign : this.verticalAlign;
  	this.offset = (offset != null) ? offset : new mxPoint();
  	this.cursor = (cursor != null) ? cursor : 'help';
  }