How to best group field values by date?

See WP_Query date parameters, especially year and monthnum, and get_post_meta().

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: 
 * Plugin URI: 
 * 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.
				if ( strtotime( $year . '-' . $month ) > 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', strtotime( $year . '-' . $month ) ),
					$this_months_food
				);
			}

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

		echo '</ul>';

		return ob_get_clean();
	}
);