artist-pen-line is a compiled Lisp function in `artist.el'.

(artist-pen-line X1 Y1)

Draw a line from last pen position to X1, Y1.
The character is replaced with the character in `artist-fill-char'.
This will store all points in `artist-key-poly-point-list' in reversed
order (I assume it is faster to cons to the beginning of the list than
to append to the end of the list, when doing free-hand drawing).