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<?php

// Set the Data Source as "OSM_geo_data"
function index_coordinates( $params, $class ) {
    if ( 'cf/OSM_geo_data' == $params['facet_source'] ) {
        $latlng = explode( ',', $params['facet_value'] ); // split the "lat,lng" into an array
        $params['facet_value'] = trim( $latlng[0] );
        $params['facet_display_value'] = trim( $latlng[1] );
    }
    return $params;
}
add_filter( 'facetwp_index_row', 'index_coordinates', 10, 2 );