How to best group field values by date?

The following PHP, when installed as a plugin for example at wp-content/plugins/shortcode-food-journal/shortcode-food-journal.php, or to your theme’s functions.php, or to a Code Snippet plugin, will enable a shortcode [food-journal] which does what I think you’re describing. It goes from the current month back to January 2000. 2000 can be changed to another starting year on the line foreach( range( date('Y'), 2000 ) as $year ) {

<?php
/**
 * Plugin Name: Shortcode: Food Journal
 * Description: Output <code>food</code> counts per month and year with shortcode <code>[food-journal]</code>.
 * Plugin Author: 𝜑δ•ℂᴹ
 * Author URI: https://pd.cm/
 * Plugin URI: https://wordpress.org/support/topic/how-to-best-group-field-values-by-date/
 * Version: 1
 */

add_shortcode(
	'food-journal',
	function( $atts, $content, $shortcode_tag ){
		ob_start();

		$base_query_args = [
			'post_type' => 'journal',
			'post_status' => 'published',
			'posts_per_page' => -1,
			'fields' => 'ids',
		];

		echo '<ul>';
		$food_by_year = [];

		// Current year back to 2000 in decending order.
		foreach( range( date('Y'), 2000 ) as $year ) {

			$food_by_year[ $year ] = 0;

			// December to January in decending order.
			foreach( range( 12, 1 ) as $month ) {
				// Skip months in the current year after the current month.
				$this_months_timestamp = strtotime( $year . '-' . $month );
				if ( $this_months_timestamp > time() ) {
					continue;
				}

				$this_months_post_ids = get_posts(
					array_merge(
						$base_query_args,
						[
							'year' => $year,
							'monthnum' => $month,
						]
					)
				);

				$this_food = $this_months_food = 0;

				if ( ! empty( $this_months_post_ids ) ) {
					foreach( $this_months_post_ids as $post_id ) {
						$this_food = (int) get_post_meta( $post_id, 'food', true );
						$this_months_food += $this_food;
						$food_by_year[ $year ] += $this_food;
					}
				}

				printf(
					'<li>Month %s, Food: %d</li>',
					date( 'F Y', $this_months_timestamp ),
					$this_months_food
				);
			}

			printf(
				'<li>Year %s, Food: %d</li>',
				$year,
				$food_by_year[ $year ]
			);
		}

		echo '</ul>';

		return ob_get_clean();
	}
);

Add another foreach after the others which iterates over $food_by_year as $year => $food.

Here:

<?php
/**
 * Plugin Name: Shortcode: Food Journal
 * Description: Output <code>food</code> counts per month and year with shortcode <code>[food-journal]</code>.
 * Plugin Author: 𝜑δ•ℂᴹ
 * Author URI: https://pd.cm/
 * Plugin URI: https://wordpress.org/support/topic/how-to-best-group-field-values-by-date/
 * Version: 2
 */

add_shortcode(
	'food-journal',
	function( $atts, $content, $shortcode_tag ){
		ob_start();

		$base_query_args = [
			'post_type' => 'journal',
			'post_status' => 'published',
			'posts_per_page' => -1,
			'fields' => 'ids',
		];

		?>
		<style>
			#food-by-month th { background-color: rgba( 10, 50, 10, .8 ); color: #fff; }
			#food-by-month tr:nth-child(even) { background-color: rgba( 10, 50, 10, .3 ); }
			#food-by-year th { background-color: rgba( 10, 10, 50, .8 ); color: #fff; }
			#food-by-year tr:nth-child(even) { background-color: rgba( 10, 10, 50, .3 ); }
		</style>
		<table id="food-by-month"><tr><th>Month</th><th>Food</th></tr>
		<?php
		$food_by_year = [];

		// Current year back to 2000 in decending order.
		foreach( range( date('Y'), 2000 ) as $year ) {

			$food_by_year[ $year ] = 0;

			// December to January in decending order.
			foreach( range( 12, 1 ) as $month ) {
				// Skip months in the current year after the current month.
				$this_months_timestamp = strtotime( $year . '-' . $month );
				if ( $this_months_timestamp > time() ) {
					continue;
				}

				$this_months_post_ids = get_posts(
					array_merge(
						$base_query_args,
						[
							'year' => $year,
							'monthnum' => $month,
						]
					)
				);

				$this_food = $this_months_food = 0;

				if ( ! empty( $this_months_post_ids ) ) {
					foreach( $this_months_post_ids as $post_id ) {
						$this_food = (int) get_post_meta( $post_id, 'food', true );
						$this_months_food += $this_food;
						$food_by_year[ $year ] += $this_food;
					}
				}

				printf(
					'<tr><td>%s</td><td>%d</td></tr>',
					date( 'F Y', $this_months_timestamp ),
					$this_months_food
				);
			}

		}

		?>
		</table>
		<table id="food-by-year"><tr><th>Year</th><th>Food</th></tr>
		<?php
		foreach( $food_by_year as $year => $food ) {
		    printf(
		        '<tr><td>%d</td><td>%d</td></tr>',
		        $year,
		        $food
		    );
		}
		echo '</table>';

		return ob_get_clean();
	}
);

For getting more field values, it’s possible to save on SQL queries by calling get_post_meta( $post_id ) (without the second or third arguments) once for each month. This will return an array of all meta for the post.

https://developer.wordpress.org/reference/functions/get_post_meta/