I was going through this article which appeared in the Notices of the AMS, and in it, there's a picture which shows a mathematical diagram drawn by Grothendieck. I would be delighted if anyone could explain what that is. Thanks in advance.
EDIT: The comments below suggest that the diagram is a dessin d'enfant. And even though I am by no means an expert, I somehow feel that it may not be one. I'd love some clarification on how one may call it so. Questions I want to ask now, are
1) Is the diagram really a dessin? Or is it something else?
2) Do the shaded parts of the diagram signify something? Are there any special types of dessins which have a similar a structure?