I’m trying to figure this out as well for a portfolio theme i’m building.
I found out how to add the category name to the “item” divs(below) but I’ve yet to find out how to add “data-filter” into the links displayed by wp_list_categories output.
function plain_category(){
$cat_plaintext = strip_tags( get_the_term_list($post->ID, 'TAXONOMY_NAME', '', ', ', '' ) );
$cat_plaintext = strtolower($cat_plaintext);
$cat_plaintext = str_replace(' ','-',$cat_plaintext);
echo $cat_plaintext;
}
and then just add
<div class="isotope-item <?php plain_category(); ?>">
content in here
</div>