$resultset= $modx->db->query("SELECT FROM_UNIXTIME( `publishedon` , '%Y' ) AS `years` , COUNT( `id` ) AS `articles` FROM " . $modx->getFullTableName('site_content') . " GROUP BY `years`");
foreach ($modx->db->getRow($resultset) as $row) {
    $array[]= $row;
}