It turns out the second and third artists, though unknown to me, were real and easily searchable. But I’m still unsure what their panes here are meant to say about their ideas or careers. And since they may not be commonplace names for many, this seems still a CIDU.
Comics I Don't Understand 
Nope, never heard of Kosuth nor Manzoni.
Whatever the association between Manzoni and the stall, we can suppose that fulfills the quota of bathroom humor and explains why Duchamp was not shown with his Fountain.
Manzoni takes 5 seconds to the Tate Gallery – https://www.tate.org.uk/art/artworks/manzoni-artists-shit-t07667
Yep! Ugh!
Okay, I’ve looked up all three artists, and seen the things being referenced.
Now, what’s the joke?
And Kosuth did a piece involving chairs.
All three are “conceptual artists” and are shown here coming up with some concepts.
I take it this is not the same Manzoni that Giuseppe Verdi wrote a requiem for.
Conceptual comic. :-)
@ Pete – Toilet humor.
@ L.F. (8) – I agree. Atkinson frequently uses this “drawing parallels” contruction on a random collection of artists or authors. It works best when a majority of the references are sufficiently well-known, or at least worthwhile to discover. Conversely, if the punchline depends on a so-called “concept artist” whose most memorable work (at least according to this comic) was a piece of excrement, one has to wonder whether the strip itself has exactly the same intrinsic value.
Three conceptual artists, and where they got their concepts.
Duchamp has a pipe, and and is thinking about the concept that a picture of a pipe isn’t a pipe.
Kosuth is looking at a chair, and thinking about the differences and similarities between a chair, a picture of a chair, and a written description of a chair.
Let’s try not to think too much about what Manzoni is thinking about.
I think for Duchamp the leading reference, besides the pipe, is the bicycle wheel — he used one as a “ready-made”.
https://www.moma.org/collection/works/81631
What the third one is contemplating seems to express my opinion of all three.
What bothers me – maybe because I am an accountant – is the middle and right hand drawings should be reversed so that they all go in date order.
Nice point, Meryl. But I guess he wanted to leave Manzoni for last as the ugly subject makes it a good “capper”.
@ Mitch (14) – I think there is an “R” missing from the final word of that comment.