docs: public static Optional<Cell> percentageChange(double oldAmount, double newAmount) {
      double delta = newAmount - oldAmount;
      double percentage = delta / oldAmount * 100.0;
      if (percentage == 0.0) {
        return Optional.absent();
      } else {
        String sign = "";
        if (percentage > 0.0) {
          sign = "+";
        }
        Cell cell = new Cell(String.format("%s%.2f%%", sign, percentage));
        // set color to red if percentage drops, to green if percentage increases
        if ("".equals(sign)) {
          cell.setColor(TextColor.RED);
        } else {
          cell.setColor(TextColor.GREEN);
        }
        return Optional.of(cell);
      }
    }