unpivot.gs

/**
 * Unpivots the given data
 * More: https://stackoverflow.com/questions/24954722/how-do-you-create-a-reverse-pivot-in-google-sheets
 *
 * @return Unpivoted data from array
 * @param {object[][]} arr 2D Input Array
 * @param {object[][]=} headers [optional] Custom headers for output
 * @customfunction
 */
function unpivot(arr, headers) {
  const custHeader = arr.shift();
  custHeader.shift();
  const out = arr.flatMap(([sale, ...qty]) =>
    qty.map((num, i) => [custHeader[i], sale, num])
  );
  if (headers) out.unshift(headers[0]);
  return out;
}