Giotto's World Champion "O"

There's a (probably apocryphal) story that the Pope was thinking of commissioning some painting or other and sent a messenger to Giotto in order to get some sort of proof of his being able to draw. Giotto apparently turned from what he was doing, drew a perfect circle freehand on presumably some sort of paper and handed it to the messenger as proof of his skill. Now you can be world champion circle drawer!