Navigo and sub-categories

Hi!
I downloaded and installed the plugin Navigo. It lets you display only children of a particular page, while all the other sub-pages collapse. Say, if you have

Articles
-Music
-Movies
Reviews
-Music
-Movies

and you go see the “movies reviews” page, you will see:

Articles
Reviews
-Music
-Movies
–Actual pages of movies reviews

While if you go to Music Articles page, you’ll see

Articles
-Music
–Actual music articles
-Movies
Reviews

That’s great! And it was exactly what I was looking for. The point is, I would like to have this too for my categories and sub-categories tree, but as far as I can see and read out there, there’s no way to do it. I mean, with “child_of” and “exclude” you can fine tune the display of the various sub-categories, but the point is: how can I add code to the cats showing in the sidebar in the template, so that it shows only the MAIN list of categories and the CHILDREN of the category (or sub-category) selected, without having to edit every single sidebar in every single page?

Let’s say you have this category tree:

Blog
News
-Music
-Movies
Plug
-Cool sites
-Other blogs

And let’s say you are in the main homepage. I would like to show only

Blog
News
Plug

But if you select the “News” category and you go to that page, I would like to show:

Blog
News
-Music
-Movies
Plug

And so on. Can anyone of you help me understand how to achieve that? Can you use the internal variable ‘$cat_ID’ somewhere or there is any plugin available to do that?

Thank you very much in advance!

-abietto-