Unique visitors per article with extra labels

I have an issue. I need to find where are or how set stats of unique visitors per single article.
E.g. I have a website.com/category-x/article-y and I want to count unique visitors, who seen this article.

Second thing is that I want to add extra label for grouping or sorting those stats.
E.g. category name or id, author.

Are those options available in Piwik features or must I modify tracking code?
How can I do this?