Customising arrows"
In ggarrow: Arrows for 'ggplot2'

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

There are a few ways in which one can tweak the display of arrow ornaments in ggarrow. This vignette goes through a few methods for making new arrow ornaments, from basic xy-coordinates to function factories and using the ornaments in scales.

plot_ornament <- function(ornament) {
  # Plot ornament
  par(pty = "s")
  plot(ornament, type = "n", xlim = c(-1, 1), ylim = c(-1, 1))
  polygon(ornament[, "x"], ornament[, "y"])

  # Incoming line
  lines(ggarrow:::debug_notching(orn, 0.2), col = 2)
  # Length measurement
  lines(x = c(0, 1), y = c(0, 0), col = 4)
  text(x = 0.5, y = 0.1, labels = "Length", col = 4)
  text(x = -0.5, y = 0, labels = "Incoming line", col = 2)
}

New ornaments

In principle, extending ornaments is as straightforward as being able to construct a polygon in an xy-coordinate matrix with x as the first column and y as the second column.

my_ornament <- function(n = 5) {
  t <- seq(0, 2 * pi, length.out = n * 2 + 1)[-(n * 2 + 1)]
  l <- rep(c(1, 0.4), length.out = length(t))

  cbind(
    x = cos(t) * l,
    y = sin(t) * l
  )
}

We can inspect the coordinates of our new ornament. It is ggarrow's convention that the line joins the ornament at the (0,0) coordinate. Also, the tip of the ornament is expected to be at the (1,0) coordinate.

orn <- my_ornament(5)

plot_ornament(orn)

You can now use your ornament as the arrow_head, arrow_fins and arrow_mid arguments. It just listens to the usual arguments like length_{head/fins/mid}, resect_{head/fins} and mid_place and scales with the line width (if the length is not an absolute unit).

library(ggarrow)

ggplot(data = data.frame(x = c(0, 1)), aes(x = x)) +
  geom_arrow(aes(y = c(1, 3)), arrow_head = orn, resect = unit(2, "cm")) +
  geom_arrow(aes(y = c(2, 2)), arrow_fins = orn, length_fins = unit(1, "cm")) +
  geom_arrow(aes(y = c(3, 1)), arrow_mid  = orn, mid_place = c(0.33, 0.66), 
             linewidth = 2)

Ornament factories

Sometimes, you might want to know more about the context in which you're drawing the ornament than can't be known before plotting. For example, let's say we wanted to add half the star we made above an arrowhead. When doing this it is immediately clear that the connection with the line looks awful.

half_star <- orn[orn[, "y"] >= 0, ]

ggplot(data.frame(x = c(0, 1), y = c(1, 1)), aes(x, y)) +
  geom_arrow(arrow_head = half_star, linewidth = 3)

If we know the linewidth in advance, you might nudge it manually. Because the default length_head is 4 and the we set the linewidth is 3, the arrowhead will get a size of 3 * 4 = 12 mm.

magic_number <- 0.7528125
half_star[, "y"] <- half_star[, "y"] - (1.5/12) * magic_number

ggplot(data.frame(x = c(0, 1), y = c(1, 1)), aes(x, y)) +
  geom_arrow(arrow_head = half_star, linewidth = 3)

The way to solve this, is to use a function factory. This method is much more involved, so be forewarned. First, if we just write a function that does as we did before, you might notice a tiny star at the end of the line as a few pixels.

half_star <- function(n = 5) {
  ornament <- my_ornament(n)
  function(...) {
    half <- ornament[ornament[, "y"] >= 0, ]
    half
  }
}

ggplot(data.frame(x = c(0, 1), y = c(1, 1)), aes(x, y)) +
  geom_arrow(arrow_head = half_star(5), linewidth = 3)

That is because the output of the function factory gets interpreted as millimetres. To be responsive to what length_head is being passed around, you should multiply your output with the length. length is one of the parameters that the function produced by the factory can receive. Doing this gives more reasonable output, but we can now see that the half-star extends beyond the path's end.

half_star <- function(n = 5) {
  ornament <- my_ornament(n)
  function(length, ...) {
    half <- ornament[ornament[, "y"] >= 0, ]
    half * length
  }
}

ggplot(data.frame(x = c(0, 1), y = c(1, 1)), aes(x, y)) +
  geom_arrow(arrow_head = half_star(5), linewidth = 3)

To control how much the line should be cut back, you can set the 'resect' attribute on your output. For this shape, we should probably resect the line by exactly the length parameter we get. Now the alignment looks as it should.

half_star <- function(n = 5) {
  ornament <- my_ornament(n)
  function(length, ...) {
    half <- ornament[ornament[, "y"] >= 0, ]
    half <- half * length
    attr(half, "resect") <- length
    half
  }
}

ggplot(data.frame(x = c(0, 1), y = c(1, 1)), aes(x, y)) +
  geom_arrow(arrow_head = half_star(5), linewidth = 3)

Lastly, to fix the actual problem we were trying to solve, we can nudge the y-coordinates by half the linewidth. width is a parameter the produced function can receive that represents the line width. Now, it looks like it should.

half_star <- function(n = 5) {
  ornament <- my_ornament(n)
  function(length, width, ...) {
    half <- ornament[ornament[, "y"] >= 0, ]
    half <- half * length
    half[, "y"] <- half[, "y"] - 0.5 * width
    attr(half, "resect") <- length
    half
  }
}

df <- expand.grid(x = c(0, 1), width = 1:4)

ggplot(df, aes(x, width, linewidth = I(width), group = width)) +
  geom_arrow(arrow_head = half_star(5)) +
  ylim(0, 5)

Besides width and length, the inner function can also receive resect. Because functionality might be expanded in the future, the last argument to the inner function should be ....

Scales

The discrete scales in ggarrow can take a mixed list of things that may define an arrow. That way, you can just put your own ornaments in a list to have it become part of the scale.

p <- ggplot(whirlpool(5), aes(x, y, group = group)) +
  coord_equal()

p + geom_arrow(aes(arrow_head = group), resect = 5) +
  scale_arrow_head_discrete(
    values = list("head_wings", orn, "fins_feather", orn, "cup"),
  )

If you start your function name with the arrow_-prefix, the ornament can be automatically found if available in the global environment.

arrow_star <- function(n = 5) {
  my_ornament(round(n))
}

p + geom_arrow(aes(arrow_head = group), resect = 1) +
  scale_arrow_head_discrete(
    values = c("head_wings", "star", "fins_feather", "star", "cup"),
  )

While not always very easy to figure out, as different arrowheads are discrete, one can in theory also apply a continuous scale to arrows. Please note that I sneaked in a round() in the function above, this is so that we can demonstrate a continuous scale with the star.

If we have something about our arrowhead that may vary in number, like an angle, or some size or in this example, the number of points on a star (though not truly continuous), we can use scale_arrow_head_continuous() to map our variable to the arrowhead. We should give the function we created as the generator argument. The variable part of our function argument should be provided as map_arg, and the range of values it can take on should be provided as range.

p + geom_arrow(aes(arrow_head = as.integer(group)), resect = 5) +
  scale_arrow_head_continuous(
    generator = arrow_star, map_arg = "n",
    range = c(3, 7)
  )