private function renderCell($columnNumber, $value, $filler, $style = '')
    {
        $output = [];
        $width = $this->getWidth($columnNumber);
        $output[] = $filler;
        while ($this->length($value) < $width) {
            $value .= $filler;
        }
        $output[] = Style::applyStyle($value, $style);
        $output[] = $filler;
        return implode('', $output);
    }