The respected Columbia Journalism Review holds forth on the subject of journalism through the comics medium, in a mostly well-done survey article of important artists and various issues. The only unsatisfying element in the essay is its vague discussion how journalism in comics form developed, a question for which it's admittedly difficult to find an answer. A lot of newspaper cartooning and illustration served a similar function for decades, and you can see comics-type reports in a lot of magazines in the 1930s and 1940s. They may not have had a direct effect on people like Joe Sacco, who may have been more personally influenced by personal essayists like Harvey Pekar and Robert Crumb, or historians like Jack Jackson. I still have to imagine such comics and cartooning might have enjoyed a greater role in shaping a place for today's comics of this type than raise-the-medium popular hits like Dark Knight Returns.
From an Art Spiegelman selection included in the article.