code: function(i, coordSystem) {
      if ("aa" === coordSystem) {
  	return String.fromCharCode(CODE_a + --i);
      }
      else { // "A1" (default)
  	var skipI = i >= 9 ? 1 : 0;
  	return String.fromCharCode(CODE_A + --i + skipI);
      }
  }
docs: Defines horizontal label.
  
  @param {number} i index of column
  @param {string} coordSystem ("A1" or "aa")
  @returns {string}