The \line command draws a line of the specified length and slope.
The slope is determined by (x-slope,y-slope) which are signed
integers of magnitude less than or equal to 6 and which have no common
divisor except for plus or minus one; For example (1,0) is a horizontal
line; (1,1) gives a slope of
45 degress; (0,1) is a vertical line; (-1,1) is 135 degrees.
The horizontal extent of the line is given by the
length parameter, except in the case of a vertical line in
which case length specifies the vertical height.