private function getOperatingSystemsCountFromSessions(Carbon $start, Carbon $end)
    {
        return $this->getVisitorsFilteredByDateRange($start, $end)
            ->filter(function (Visitor $visitor) {
                return $visitor->hasDevice();
            })
            ->transform(function (Visitor $visitor) {
                return $visitor->device;
            })
            ->groupBy('platform')
            ->transform(function (Collection $items, $platform) {
                return [
                    'platform' => $platform,
                    'count'    => $items->count(),
                ];
            })
            ->sortByDesc('count');
    }