ggplot2 plot an angle between two lines

Question

I would like to plot an angle between two lines using ggplot2, meaning something similar to the bold red line in the plot below. Is there an easy solution to this?

Data and code to make the plot without the red line:

library(tidyverse)

df <- tibble(
  line = c("A", "A", "B", "B"),
  x = c(1, 5, 1, 3),
  y = c(1, 3, 1, 5))

ggplot(
  df, aes(x, y, group = line))+
  geom_path()

Answer 1

have a look at geom_curve, e.g. :

ggplot(  df, aes(x, y, group = line))+
  geom_path() +
  geom_curve(aes(x = 1.5, y = 2, xend = 2, yend = 1.5), curvature = -0.5, color = "red", size = 3)

You will have to tweak it a bit to use it in a more robust, automatic way, for example:

red_curve <- df %>%
  group_by(line) %>%
  summarise( avg_x = mean(x),
             avg_y = mean(y))

ggplot(  df, aes(x, y, group = line))+
  geom_path() +
  geom_curve( data = red_curve, aes(x = avg_x[1], y = avg_y[1], xend = avg_x[2], yend = avg_y[2]), curvature = 0.5, color = "red", size = 3)

Answer 2

Here is a solution with geom_arc of the ggforce package.

library(ggplot2)
library(ggforce)

angle <- function(p, c){
  M <- p - c
  Arg(complex(real = M[1], imaginary = M[2]))
}

O <- c(1,1)
P1 <- c(5,3)
P2 <- c(3,5)
a1 <- angle(P1, O)
a2 <- angle(P2, O)

df <- data.frame(
  line = c("A", "A", "B", "B"),
  x = c(1, 5, 1, 3),
  y = c(1, 3, 1, 5)
)

ggplot(df, aes(x, y, group = line)) +
  geom_path() + 
  geom_arc(aes(x0 = 1, y0 = 1, r = 1, start = a1, end = a2), 
           color="red", size = 2, inherit.aes = FALSE)

The arc does not look like a true arc circle. That's because the aspect ratio is not set to 1. To set the aspect ratio to 1:

ggplot(df, aes(x, y, group = line)) +
  geom_path() + 
  geom_arc(aes(x0 = 1, y0 = 1, r = 1, start = a1, end = a2), 
           color="red", size = 2, inherit.aes = FALSE) + 
  coord_fixed()