Combine two facets’ data source into one facet

<?php

/**
 * This will trick FacetWP's indexer into indexing "tags_list" values for the "categories_new" facet
 *
 * Re-index after adding the following code into your (child) theme's functions.php
 */
add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'tags_list' == $params['facet_name'] ) {
        $params['facet_name'] = 'categories_new';
    }
    return $params;
}, 99, 2 );