code: function(j, coordSystem, size) {
      if ("aa" === coordSystem) {
  	return String.fromCharCode(CODE_a + --j);
      }
      else { // "A1" (default)
  	return (size - --j).toString();
      }
  }
docs: Defines vertical label.
  
  @param {number} j index of row
  @param {string} coordSystem ("A1" or "aa")
  @param {number} size the grid base (9, 13, 19)
  @returns {string}