LOL, I agree with what Farmerdill said!
Why even bother with grafting? Just plant a tomato plant and a potato in the same hole... voila! potatoes underground and tomatoes above ground. I tried this some years back, thinking it sounded like a great idea; but (newsflash!) the potatoes are ready to be harvested long before the tomato plants are ready to be pulled out. Not that it would really hurt anything to leave them in the ground, I guess, as long as you remember they are there, and don't want "new potatoes." Definitely better than paying money for a "special" Tomtato" (or whatever they're calling them)!