4 columns: x y length angle
The keywords with arrows may be followed by inline arrow style properties, a reference to a predefined arrow style, or arrowstyle variable to load the index of the desired arrow style for each arrow from a separate column.
length > 0 is interpreted in x-axis coordinates. -1 < length < 0 is interpreted in horizontal graph coordinates; i.e. |length| is a fraction of the total graph width. The program will adjust for differences in x and y scaling or plot aspect ratio so that the visual length is independent of the orientation angle.
angle is always specified in degrees.
Copyright 1986 - 1993, 1998, 2004 Thomas Williams, Colin Kelley
Distributed under the gnuplot license (rights to distribute modified versions are withheld).