XKCD has a long history of citing their work, why can't an effective argument be made with visuals? I don't understand why the medium of the argument matters here.

There is no argument made, it's a comic strip.

There is definitely an argument made. It might not be in the form that you're looking for, but the argument is there. The graph clearly points to a high correlation between the industrial revolution and a fast and huge (relative to other changes) rise in temperatures. How is this not an argument?

Yes, very clearly, sure. You don't even know the very basics of scientific methodology, statistics, interpreting data, or critical thinking.

It's actually a graph presented in a great way. Read it

Comic or not - what a great infographic

It's a really simple explanation to refute deniers, not a scientific paper