CountEmail.php

<?php
/**
 * @author Roman Ožana <[email protected]>
 */
class CountEmail {

	/**
	 * @return MailFrom
	 */
	public static function init() {
		return new self;
	}

	public function __construct() {
		add_action('admin_menu', [$this, 'adminMenu']);
		add_filter('wp_mail', [$this, 'wp_mail']);
	}

	public function adminMenu() {
		add_submenu_page(
			'index.php', 'Email stats', 'Email stats', 'manage_categories', __CLASS__, [$this, 'countEmail']
		);
	}

	/**
	 * Page with results
	 */
	public function countEmail() {
		if (!current_user_can('manage_categories')) {
			wp_die(__('You do not have sufficient permissions to access this page.'));
		}

		$counter = self::getCounter();

		$days = [['Day', 'Send emails']];
		ksort($counter->day);
		foreach (array_slice($counter->day, -14, null, true) as $day => $count) {
			$date = \DateTime::createFromFormat('z', $day)->format('j.n.');
			$days[] = [	$date, $count];
		}

		$months = [['Month', 'Send emails']];
		ksort($counter->month);
		foreach ($counter->month as $month => $count) {
			$date = \DateTime::createFromFormat('n', $month)->format('U');
			$months[] = [date_i18n('F', $date), $count];
		}
		require __DIR__ . '/CountEmail.phtml';
	}

	/**
	 * Return or update counter
	 *
	 * @param bool $update
	 * @param null $time
	 * @return mixed|void
	 */
	public static function getCounter($update = false, $time = null) {
		list($day, $week, $month, $year) = array_map('intval', explode('|', date('z|W|m|Y', $time ? : time())));

		$option = 'count_email_' . $year;
		$counter = get_option($option, (object)['day' => [], 'week' => [], 'month' => [], 'total' => 0]);

		if ($update) {
			$counter->day[$day] = isset($counter->day[$day]) ? $counter->day[$day] + 1 : 1;
			$counter->week[$week] = isset($counter->week[$week]) ? $counter->week[$week] + 1 : 1;
			$counter->month[$month] = isset($counter->month[$month]) ? $counter->month[$month] + 1 : 1;
			$counter->total++;

			update_option($option, $counter); // save counter
		}

		return $counter;
	}

	/**
	 * Update counter on email send
	 *
	 * @param $email
	 * @return mixed
	 */
	public static function wp_mail($email) {
		self::getCounter(true);
		return $email;
	}
} 

CountEmail.phtml

<style type="text/css">
	.graph {
		width: 100%;
		max-width: 1024px;
		height: 500px;
		margin: 10px 0;
		border: 1px solid #ddd;
		background: #fff
	}
</style>
<div class="wrap">
	<h1>Send emails</h1>

	<p>Total send <strong><?= $counter->total ?></strong> emails this year</p>

	<script type="text/javascript" src="https://www.google.com/jsapi"></script>

	<script type="text/javascript">
		google.load("visualization", "1", {packages: ["corechart"]});
		google.setOnLoadCallback(drawChart);

		function drawChart() {
			<? if (count($days) > 1) { ?>
			var days = google.visualization.arrayToDataTable(<?= json_encode($days)?>);
			var chart = new google.visualization.ColumnChart(document.getElementById('day_chart_div'));
			chart.draw(days, {});
			<? } ?>

			<? if (count($months) > 1) { ?>
			var months = google.visualization.arrayToDataTable(<?= json_encode($months)?>);
			var chart2 = new google.visualization.ColumnChart(document.getElementById('moth_chart_div'));
			chart2.draw(months, {});
			<? } ?>
		}
	</script>


	<h2>Send email per month</h2>

	<div id="moth_chart_div" class="graph"></div>

	<h2>Send emails in last 14 days</h2>

	<div id="day_chart_div" class="graph"></div>

</div>