WordPress: 投稿数を年別に集計する

add_filter( 'getarchives_join', 'lf_getarchives_join', 10, 2 );

$out = wp_get_archives( [
	'type'            => 'yearly',
	'show_post_count' => true,
	'echo'            => false,
	'order'           => 'ASC',
] );

remove_filter( 'getarchives_join', 'lf_getarchives_join' );

// 生成されたリストから年と投稿数を切り出す
preg_match_all(
    '|\>(\d+)\</a.+\((\d+)\)\</li|',
    $archives,
    $matches
);

// 年を数値に変換
$years = array_map( 'intval', $matches[1] );

// 年別投稿数を数値に変換して累積値を取得
$counts = [];
foreach ( $matches[2] as $i => $match ) {
    $counts[] += (int)$match + ( $counts[$i-1] ?? 0 );
}

function lf_getarchives_join( $sql_join, $parsed_args ) {
	return "
	JOIN wp_term_relationships AS tr ON wp_posts.ID = tr.object_id 
	JOIN wp_term_taxonomy AS tt ON tr.term_taxonomy_id = tt.term_taxonomy_id AND tt.taxonomy = 'category'
	JOIN wp_terms AS t ON tt.term_id = t.term_id AND t.name='list'
	";
}

$query = "SELECT YEAR(post_date) AS year, COUNT(post_date) AS count
FROM wp_posts AS p
JOIN wp_term_relationships AS tr ON p.ID = tr.object_id 
JOIN wp_term_taxonomy AS tt ON tr.term_taxonomy_id = tt.term_taxonomy_id AND tt.taxonomy = 'category'
JOIN wp_terms AS t ON tt.term_id = t.term_id AND t.name='list'
WHERE p.post_status='publish' AND p.post_type='post'
GROUP BY YEAR(post_date)";
$publists = $wpdb->get_results( $query );

// グラフの横軸（年）と縦軸（累積投稿数）データを取得
foreach ( $publists as $i => &$publist ) {

    // 前年の投稿数（初年度の前年は0）を加算
    $publist->count += ( $publists[$i-1]->count ?? 0 );

    $years[]  = $publist->year;
    $counts[] = $publist->count;
}

// 全クエリ共通
$query_base = [
    'post_type'      => 'post',
    'post_status'    => 'publish',
    'category_name'  => 'list',
];

foreach( [ 'ASC', 'DESC' ] as $order ) {
    $query = new WP_Query( $query_base + [
        'posts_per_page' => 1,
        'orderby'        => 'date',
        'order'          => $order,
    ] );

    // 投稿日の最初の4文字を数値化して投稿年を取得
    $date = $query->posts[0]->post_date;
    $year[$order] = intval( substr( $date, 0, 4 ) );
}

// 投稿数の年別累計値を取得
$years = $counts = [];
for( $y = $year['ASC']; $y <= $year['DESC']; $y++ ) {
    $query = new WP_Query( $query_base + [
        'posts_per_page' => 1, // ヒット件数(found_posts)だけわかればよいので実データは最小
        'fields'         => 'ids', // ID のみ
        'year'           => $y,
    ] );

    $years[]  = $y;
    $counts[] = ( $counts[$y-1] ?? 0 ) + $query->found_posts;
}