This question borders philosophy, so I would be happy to move it somewhere else if this forum is inappropriate.
While doing some 3D function plotting, it occurred to me that I can plot any shape that I can think of (theoretically that is), including all observable plain objects. My question is: Is there any physical object the look of which one cannot plot as a mathematical function? Note that the difficulty of such a task is beside the point; I am merely concerned with the decidability of this issue.