I found some books which are available from dozens of online bookshops but which are not in this visualisation. Perhaps they're not yet in any library that feeds into worldcat.org, though some of them were about five years old.
Did you search by title or ISBN? If you search by title, the search goes through Google Books, which is very incomplete (since I didn't build a search database myself). If you put in an ISBN13 directly, you'll find a lot more books are included (I'd say you can only find 10-30% of books via the Google Books API)
It's a bit misleading I guess the way I added that feature.
