Description
https://bib.schema.org/BookFormatType lists the following types:
EBook, Hardcover and Paperback are publication formats for various types of content, mostly for (written) text or text with illustrations. It might be images and nothing else. In other words, these types describe an outer form and not the content. The primary mode for the content presentation is visual, but otherwise content is undefined. (Some books include multimodal content like haptic elements or sound, e.g. in children's books, but that is beside my point here.)
An Audiobook is for spoken text, obviously using auditory modality. I can understand that is practical to list it as a BookFormatType even though it is primarily used for text and cannot adequately convey images. (It can convey sound beyond spoken word, obviously - sound effects or music.)
A graphic novel is a longform comic, a specific kind of content or medium (in which text and images combine in specific ways). A graphic novel can be published in print (Hardover or Paperback) or electronic format (EBook). To me, it does not make sense to conceptualize as a BookFormatType next to the others. It's like listing "artbook" or "short story collection" on the same level as EBook, Hardcover, Paperback. That's mixing type of content with output formats. Therefore, I think graphic novel should be removed here.
(I'm new to this, apologies if I made mistakes.)