If you were to flatten the cylinder out after you had cut it, the shape of the mitre line would be a segment of a sine curve.
If it were a complete cylinder, the amplitude of the sine curve would be the difference between the longest and the shortest length along the pipe, and the period would be the circumference of the cylinder. If you cut straight across, it would be a straight line (a sine curve with an amplitude of 0).
(Before CAD we used some nice drafting techniques to get the shape of the curve.)
Rich Kleinschmidt Bishop Brady High School Concord, NH email@example.com