The \vector command draws an arrow of the specified length and slope
with the arrow head at the opposite end of the line from the
reference point (see \put).
The slope is determined by (x-slope,y-slope) which are signed
integers of magnitude less than or equal to 4 and which have no common
divisor except for plus or minus one; For example (1,0) is a horizontal
arrow pointing right; (1,1) gives a slope of
45 degress; (0,1) is a vertical arrow pointing up; (-1,1) is at 135 degrees.
The horizontal extent of the arrow is given by the
length parameter, except in the case of a vertical arrow in
which case length specifies the vertical height.