My bosses force me to add an author to my paper who didn't contribute to it
There are a couple of clauses in your question which you may want to reconsider.


I worked completely independent &mdash; did you? You wrote a paper working on your own, which is indeed a big achievement. However, you've been working with data, and the data have been obtained by A in the lab run by X. This definitely does not sound like a completely independent work to me.
X contributed nothing to the work &mdash; did s/he not? They set up  and run a lab, where the data have been obtained. Could you write the same paper without the data A collected? Could A collect the data without X's lab?


The arguments above do not necessarily mean that X should co-author the paper, but this is a matter of agreement between all collaborators. In ideal world, this should be discussed before the project is started; in reality, discussions like you've had are often held too late into the process, and the frustration you're feeling is understandable. Be strong &mdash; you will be a completely independent researcher one day very soon, and hopefully you will manage your projects and relations in much more thoughtful and transparent way. Until then it is best not to waste your life in such battles.