If categories cannot be enabled in Recently Popular WordPress plugin

Recently Popular is a WordPress plugin that will let you find and display for visitors of your blog the most popular posts in time intervals you can define at will. The plugin displays popular posts at the sidebar, but it lets you also use shortcodes to display them on specific pages or posts.

If you decide to use the second method, you may find out that categories filtering (categories_enable) won’t work. Unfortunately the developer gives vague directions on how to use categories, just saying to enclose them in single quotes.

Following this most probably you write:

'categories' => 'catx', 'catz',

Then you see that it doesn’t work and you try:

'categories' => 'catx, catz',

This won’t work either. The only way to do it, which the author fails to mention in the directions of the plugin, is to enclose each category in single quotes, and all of them in double quotes. This will work:

'categories' => "'catx, catz'",