Produces a description of what arrows to add to a line.
The result can be passed to a function that draws a line,
arrow(angle = 30, length = unit(0.25, "inches"), ends = "last", type = "open")
The angle of the arrow head in degrees (smaller numbers produce narrower, pointier arrows). Essentially describes the width of the arrow head.
A unit specifying the length of the arrow head (from tip to base).
indicating which ends of the line to draw arrow heads.
whether the arrow head should be a closed triangle.