Books are challenged regularly at libraries across the country for a number of reasons and graphic novels are no exception, though the perception may be that they get attacked more because "comics are for kids!" and how could we let them read such wretched things. The American Library Association releases a list of these books every year during Banned Books Week which we're in the middle of now. Definitely check out that site to see more about it and the promotion of the idea of Freedom to Read. The list of graphic novels tends to be pretty familiar from year to year and there's a lot of local variance to things in how libraries may classify certain books by age groups as well, but the overall themes do appear to be pretty consistent.
The discussion on why each of these books is unacceptable can be pretty fascinating to have, depending on who is involved and the context of it, something we don't actually get. I don't know how many Mania readers are parents, but every parent and child is different in what is acceptable to them of course. I know that I'm in the midst of reading Bone to my kids (girls ages 8 and 10) and they adore it. I know I read Watchmen in my mid teens along with other books here such as Tank Girl and Dark Knight Strikes Again. And it's certainly understandable to make sure certain books aren't available to kids of a certain age without parental consent, but the idea to remember is that libraries are being told to make sure these books are not kept in libraries at all and are firmly removed for the reasons above.
As many comics fans may have done over the years, multiple copies of certain trades are not unusual. If you have some, make sure to donate them to your library to help promote the freedom to read for others, especially if you have these top challenged books to spare. Bring it up directly with your librarian so they know about it as well.