Making a custom arrowhead for ggplot2 using ggarrow and arrowheadr

W. Joel Schneider

Base plot for demonstrations

library(tidyverse)
# install.packages("remotes")
# remotes::install_github("teunbrand/ggarrow")
library(ggarrow)

node_radius <- .2
resection_length <- .03

d_node <- tibble(node = c("A", "B", "C"),
       x = c(0, .5, 1),
       y = c(0, sqrt(3) / 2, 0))

d_edge <- d_node |> 
  mutate(theta_next = atan((y - lag(y)) / (x - lag(x))),
         x_from = lag(x) + cos(theta_next) * (node_radius + resection_length),
         y_from = lag(y) + sin(theta_next) * (node_radius + resection_length),
         x_to = x + cos(theta_next + pi) * (node_radius + resection_length),
         y_to = y + sin(theta_next + pi) * (node_radius + resection_length)) |> 
  filter(!is.na(x_from))

p <- d_edge |>
  ggplot(aes(x = x_from, y = y_from, xend = x_to, yend = y_to)) +
  ggforce::geom_circle(aes(
    x0 = x,
    y0 = y,
    r = .2,
    fill = node
  ),
  data = d_node,
  color = NA, inherit.aes = F) +
  geom_text(aes(label = node, x = x, y = y), 
            data = d_node, 
            size = 20,
            family = "Roboto Condensed",
            inherit.aes = F) +
  coord_fixed() +
  theme_void() +
  theme(legend.position = "none") +
     scale_fill_viridis_d(
       option = "D",
       begin = .2,
       end = .8,
       alpha = .5
     )

p

The default arrows in ggplot2 are perfectly serviceable. There is the open variety:

Open arrow code

p + geom_segment(arrow = arrow())

You can specify which ends of the segment have arrowheads, whether the arrows are open or closed, the length of the arrow, and how sharp the arrow’s point is with the angle argument.

I happen to prefer a longer, sharper, closed arrow:

Closed arrow code

p + geom_segment(
  arrow = arrow(
    angle = 15,
    length = unit(8, "mm"),
    type = "closed"
))

Nevertheless, I miss the variety of arrows in available in TikZ. I particularly like the latex' arrow:

TikZ code with latex’ arrow

\usetikzlibrary{arrows}
\begin{tikzpicture}
\node[fill=violet!50!white, circle] (A) at (0,0) {A};
\node[fill=cyan!80!black, circle] (B) at (1,1.732) {B};
\node[fill=green!40!black!40, circle] (C) at (2,0) {C};
\path[->,
    draw,
    shorten >=2pt,
    shorten <=2pt,
    >=latex',
    thick,
    color = black!40,
    text = black] (A) -> (B);
    \path[->,
    draw,
    shorten >=2pt,
    shorten <=2pt,
    >=latex',
    thick,
    color = black!40,
    text = black] (B) -> (C);
\end{tikzpicture}

Figure 3: The TikZ latex’ arrow

The ggarrow package

If you want more variety in drawing arrows in ggplot2, Teun van den Brand’s ggarrow package expands your limits to whatever your imagination can provide.

The default arrowhead is already a nice improvement:

Code for wings arrow (i.e, stealth arrow in TikZ)

p + geom_arrow_segment(length_head = 10)

You can play around with the sharpness of the point (offset) and the sharpness of the barb (inset). You can also add feathers.

Code for sharp barbs and feathers

p +
  geom_arrow_segment(length_head = 4,
                     arrow_head = arrow_head_wings(offset = 20, 
                                                   inset = 10), 
                     arrow_fins = arrow_fins_feather(), 
                     length_fins = 11)

Code for kite arrowhead

p +
  geom_arrow_segment(length_head = 15,
                     arrow_head = arrow_head_wings(offset = 22.5,
                                                   inset = 115),
                     arrow_fins = arrow_head_wings(offset = 22.5,
                                                   inset = 115),
                     length_fins = 15)

Figure 6: Double-headed arrow with kite arrowhead

Code for reverse kite arrowhead

p +
  geom_arrow_segment(length_head = 20,
                     arrow_head = arrow_head_wings(offset = 45,
                                                   inset = 120))

If the arrow goes too far, you can pull it back with the resect arguments.

Code for resecting an arrowhead

p +
  geom_arrow_segment(length_head = 6,
                     arrow_head = arrow_head_wings(offset = 120,
                                                   inset = 35),
                     resect_head = 2)

Figure 8: Demonstration of resecting arrowheads

There is much, much more that can be done. See ggarrow’s arrow ornament vignette for more options.

Custom Arrowheads

Not only does ggarrow offer great arrow geoms with excellent features like resection, one can create any custom arrowhead or feather that can be made with a single polygon.

The polygon generally falls between -1 and 1 on x and y, though you can plot outside those limits. In most cases, the point is at (0,1), and the line ends at (0,0):

Code

par(pty = "s")
x <- c(0,1)
y <- c(0,0)
plot(x , y, xlim = c(-1, 1), ylim = c(-1, 1))
rect(-1,-.1,0,.1)
text(-.5,0, labels = "Line")
text(1,0, labels = "Arrow Point", adj = 1.2)
polygon(ggarrow::arrow_head_wings() |> `colnames<-`(c("a", "b")))

The polygon you create should be be a 2-column matrix with named columns (e.g., x and y). Here I make a elliptical arrowhead.

Code

make_ellipse <- function(a = 1, b = .5){
  t <- seq(0,2*pi, length.out = 361)
  cbind(x = a * cos(t), y = b * sin(t)) 
}

p +
  geom_arrow_segment(length_head = 5,
                     arrow_head = make_ellipse())

The arrowheadr package

I made the arrowheadr package to make custom arrowheads quickly. Some of them are admittedly silly. However, it is now easy to make my favorite kind of arrow in ggplot2:

Code

library(arrowheadr)

p + 
  geom_arrow_segment(length_head = 5,
                     arrow_head = arrow_head_latex())

More examples:

A catenary:

Code

stlouis <- arrow_head_catenary(base_width = .25, thickness = .15)

p + 
  geom_arrow_segment(length_head = 5,
                     arrow_head = stlouis)

Figure 11: The Gateway Arch in St. Louis is shaped like a catenary, not a parabola.

The Cauchy function:

Code

p + 
  geom_arrow_segment(length_head = 5,
                     arrow_head = arrow_head_function(dt, df = 1))

Figure 12: Any function can make an arrowhead.

Razors:

Code

razors <- c(1,0,
  0,.5,
  -.35,.25,
  -.35, .21,
  0,.35,
  .90,0
  ) |> 
  v2matrix() |> 
  reflecter() 

p + 
  geom_arrow_segment(length_head = 10,
                     arrow_head = razors)

Figure 13: The reflecter function takes a set of points and makes them symmetrical.

A candle flame:

Code

candleflame <- arrow_head_wittgenstein_rod(
  fixed_point = c(-2.75, 0),
  rod_length = 3.75,
  nudge = c(1, 0),
  rescale = .95
)

p + 
  geom_arrow_segment(length_head = 12,
                     arrow_head = candleflame)

Figure 14: Wittgenstein’s rod makes some nice shapes

Using bezier curve control points:

Code

curved_arrowhead <- arrow_head_bezier(list(
  c(1,  0,
    .5, .5,
    .2, .5),
  c(.2, .5,
    .2, .1,
    -.1, .25,
    -.3, .25),
  c(-.3, .25,
    0, 0,
    -.3, -.25),
  c(-.3, -.25,
    -.1, -.25,
    .2,  -.1,
    .2, -.5),
  c(.2, -.5,
    .5, -.5,
    1,  0)
))

p + 
  geom_arrow_segment(length_head = 8,
                     arrow_head = curved_arrowhead)

Figure 15: Bezier curves can make almost anything.

Citation

BibTeX citation:

@misc{schneider2023,
  author = {Schneider, W. Joel},
  title = {Making a Custom Arrowhead for Ggplot2 Using Ggarrow and
    Arrowheadr},
  date = {2023-08-26},
  url = {https://wjschne.github.io/posts/2023-08-26-making-a-custom-arrowhead-for-ggplot2-using-ggarrow-and-arrowheadr/},
  langid = {en}
}

For attribution, please cite this work as:

Schneider, W. J. (2023, August 26). Making a custom arrowhead for ggplot2 using ggarrow and arrowheadr. Schneirographs. https://wjschne.github.io/posts/2023-08-26-making-a-custom-arrowhead-for-ggplot2-using-ggarrow-and-arrowheadr/