使用WP_Query的Wordpress短代码

Question

I'm developing a custom wp plugin for category loops. I'm trying to get posts that have multiple categories.

For instance, I want to get ONLY the posts that have BOTH categories 1 and 2 - meaning that posts that have categories 1 OR 2 don't appear like so:

[myplugin category="1,2"]

This don't appear to work, as it is displaying posts that have ONLY the first category (1).

Searched and tried a lot of ways to achieve this. My actual plugin code is this:

function pluginHandler($atts, $content = null) {
     $cat = empty($atts['category']) ? '0' : $atts['category'];
     global $post;
     $args = array(
          'posts_per_page' => -1,
                'category__and' => array($cat)
     );

     $the_query = new WP_Query($args);

     if ( $the_query->have_posts() ) :
            while ( $the_query->have_posts() ) :
                $the_query->the_post();

            ...

            endwhile;
     endif;
}

What am I doing wrong?