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.