Do I have any obligations to my PhD supervisor's requests after I have graduated?
I think it's reasonable for X to ask, but you have no obligation to say "yes". In particular, 2-3 months of weekends is a lot of time to work for free. I think that if you choose, it would be reasonable for you to politely say that this is too much, and that he needs to get somebody else to do that analysis. If this is work that you have already done a lot of, you should still be an author on the resulting paper.

If your new job is with a university, then it is worth asking your new supervisor if you can spend some time on it. They may be happy that they can get a paper with their affiliation for only a couple of weeks of work. If your new job is a commercial one, this is much less likely to help.