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?