I think you’re looking for the “conditional” get_post_type.
In your case it would be:
function orderby_post_title_int( $orderby, $query ) {
if ( 'chart' == get_post_type() ) {
return '(wp_posts.post_title+0) ASC';
}
return $orderby;
}
Or you could insert the conditional in your original function, which you know was working?
(Note if this is a custom post type archive, you’d use the is_post_type_archive() conditional instead.)