Vasari's Lives of the Painters, Sculptors and Architects tells the story of how Giotto drew a perfect circle to prove his skill. One day in April I tried to draw perfect circles whilst standing on the tube, leaning with a shoulder on the glass. There are five drawn between each of three stops on the Northern line (one, two, three), then the Victoria line (one, two, three), and then the Bakerloo line (one, two, three). In the choice of tube lines a pun about the Circle line has gone begging - perhaps next time.
And one morning going into the workshop of Giotto, who was at his labours, [Pope Benedict's courtier] showed him the mind of the Pope, and at last asked him to give him a little drawing to send to his Holiness. Giotto, who was a man of courteous manners, immediately took a sheet of paper, and with a pen dipped in red, fixing his arm firmly against his side to make a compass of it, with a turn of his hand he made a circle so perfect that it was a marvel to see it. Having done it, he turned smiling to the courtier and said, "Here is the drawing." But he, thinking he was being laughed at, asked, "Am I to have no other drawing than this?" "This is enough and too much," replied Giotto, "send it with the others and see if it will be understood." The messenger, seeing that he could get nothing else, departed ill pleased, not doubting that he had been made a fool of. However, sending the other drawings to the Pope with the names of those who had made them, he sent also Giotto's, relating how he had made the circle without moving his arm and without compasses, which when the Pope and many of his courtiers understood, they saw that Giotto must surpass greatly all the other painters of his time.
More on Giotto and the circle, and others who demonstrated skill with a simple gesture.
More drawings on the tube: Berlin U6 drawing: circle; a transcription of Caspar David Friedrich's The Abbey in the Oak Forest 1809-10; and Underground train drawings one and two - the artist and tube train making the drawing together.