I suggest the approach by Kieran Brown (2018) "Comics, Materiality, and the Limits of Media Combinations." in ImageTexT, vol 9, no. 3, is an interest text about graphic novels and intermediality.
Graphic novels are not purely visual because they may integrate some language or some form of language. In fact, they are like Maya carvings on monuments, or paintings on cups, mugs, and plates, or complex stories and predictions in the four codices that survived or escaped the Spanish autodafe in the 16th century. There is a lot of research about Maya visual arts and most researchers neglect most of the time the linguistic side of things. I am afraid "visual" is such a term that neglects the linguistic side of things. Writing is such a visual use of language.