private function axisYears(string $boundaries_csv): array
    {
        $boundaries = explode(',', $boundaries_csv);

        $axis = [];
        foreach ($boundaries as $n => $boundary) {
            if ($n === 0) {
                $date = new Date('BEF ' . $boundary);
            } else {
                $date = new Date('BET ' . $boundaries[$n - 1] . ' AND ' . ($boundary - 1));
            }
            $axis[$boundary - 1] = strip_tags($date->display());
        }

        $date              = new Date('AFT ' . $boundaries[count($boundaries) - 1]);
        $axis[PHP_INT_MAX] = strip_tags($date->display());

        return $axis;
    }