This option indicates how to draw arrowheads. The shape argument must be
a list with three elements, each specifying a distance. The first
element of the list gives the distance along the line from the neck of
the arrowhead to its tip. The second element gives the distance along
the line from the trailing points of the arrowhead to the tip, and the
third element gives the distance from the outside edge of the line to
the trailing points.
This option indicates how to draw arrowheads. The shape argument must be a list with three elements, each specifying a distance. The first element of the list gives the distance along the line from the neck of the arrowhead to its tip. The second element gives the distance along the line from the trailing points of the arrowhead to the tip, and the third element gives the distance from the outside edge of the line to the trailing points.