A skin graft is a patch of skin that is removed by surgery from one area of the body and is attached to another area. "Skin graft" sounds like "skin graph". A tattoo is a type of skin modification involving ink. A graph is a diagram showing the relation between variable quantities, typically of two variables. Scientists often use or analyze graphs to understand trends. This pun is playing on the word "graph" because it sounds like "graft" as in "skin graft" while referring to a scientist wanting a tattoo.
A scientist who wanted a tattoo got a skin graph .