pedant
Explanation
The Joke
A woman confronts her partner, telling him, "Honey, you're too pedantic. It's driving us apart. Here, I made a graph of how pedantic you've become." She holds up a chart intended to illustrate her point. The man, rather than engaging with the emotional substance of her complaint, immediately zeroes in on the graph itself -- presumably finding fault with its axes, labeling, methodology, or some other technical detail. His horrified/critical expression while staring at the graph confirms that he is being pedantic about the very graph that was made to show him he is too pedantic.
The comic is a perfect self-referential loop: the act of being shown evidence of pedantry triggers more pedantry. The woman's attempt to communicate an emotional concern using data backfires because she has chosen exactly the wrong medium for a pedant -- a graph he can critique.
The Humor
The humor is in the beautiful irony of the situation. A pedant cannot help but be pedantic, even when the subject of the pedantry is his own pedantry. The woman has inadvertently proven her own point more effectively than she intended, but in a way that will never actually resolve the argument. Anyone who has lived with an overly detail-oriented person will recognize the frustration of trying to make an emotional point only to have the other person fixate on some irrelevant technical detail of how the point was presented.