I'm not sure exactly as I haven't read it. But generally, a graphic novel often tells a story through a combination of illustrations and text. It might be about a spooky or mysterious story related to a coffin, perhaps involving a character's journey with a coffin or something hidden in it.
Well, without having read 'the coffin graphic novel' specifically, it could be centered around the symbolism of a coffin. Maybe it explores themes like death, the afterlife, or secrets that are buried along with the dead. It could also be a horror - themed story where the coffin is a source of danger or holds some sort of malevolent entity.