The scan-line conversions of the drawing functions are very much host-dependent. In other words, you cannot assume that, for example
(draw-point port x y)
has exactly the same effect on all machines. Some machines might put pixels down and to the right of integer coordinates
while others may center the pixel at
See also the LispWorks
CAPI Reference Manual
(which draws a circle) and
(which draws an ellipse) and other drawing functions.