Scholastic defines a graphic novel in the following manner:

To be considered a graphic novel, rather than a picture book or illustrated novel, the story is told using a combination of words and pictures in a sequence across the page. Graphic novels can be any genre, and tell any kind of story, just like their prose counterparts.

This market can be considered to represent just the first book, Diary of a Wimpy Kid, or the entire series.

Basically, if you owned a bookstore in which you only had two sections: "graphic novels" and "other" - which section of your store do you place Diary of a Wimpy Kid?