Add custom text in specific points of axis using ggplot2

Question

I have creted the plot below and I would like to add custom text in the x-axis in the points where those 2 vertical lines cross the x-axis like in the example picture below. The first βi and the second βj. How is that possible?

# Set x-axis values
theta <- seq(from = -10, to = 10, by = 0.001)

B_i <- 1
B_j <- -1
P_item0_rasch <- NULL
P_item1_rasch <- NULL
P_item2_rasch <- NULL
for (i in 1:length(theta)){
  P_item0_rasch[i] <- (exp((theta[i])))/(1+(exp((theta[i]))))
  P_item1_rasch[i] <- (exp((theta[i]-B_i)))/(1+(exp((theta[i]-B_i))))
  P_item2_rasch[i] <- (exp((theta[i]-B_j)))/(1+(exp((theta[i]-B_j))))
}

df <- data.frame(theta = rep(theta, 3),
                 P_item_rasch = c(P_item0_rasch, P_item1_rasch, P_item2_rasch),
                 number = factor(rep(1:3, each = length(theta))))

library(ggplot2)

ggplot(df, aes(theta, P_item_rasch)) +
  geom_line(aes(color = number)) +
  lims(x = c(-6, 6)) +
  # Line between curves
  geom_segment(x = -1, xend = 1, y = 0.5, yend = 0.5, lty = 2) +
  # Optional line on left
  geom_segment(x = -Inf, xend = -1, y = 0.5, yend = 0.5, lty = 2) +
  # Lower lines
  geom_segment(data = data.frame(theta = c(-1, 0, 1), P_item_rasch = rep(-Inf, 3)),
               aes(xend = theta, yend = 0.5), lty = 2) +
  # Upper lines
  #geom_segment(data = data.frame(theta = c(-1, 0, 1), P_item_rasch = rep(Inf, 3)),
  #            aes(xend = theta, yend = 0.5), lty = 2) +
  scale_color_manual(values = RColorBrewer::brewer.pal(4, "Set1")[-1]) +
  theme_classic() +
  theme(legend.position = "none")

do you want to keep the other numbers in the x axis?

Allan Cameron
– Allan Cameron

2020-12-06 23:00:35 +00:00
Commented Dec 6, 2020 at 23:00 — Allan Cameron
– Allan Cameron, Commented Dec 6, 2020 at 23:00

Allan Cameron · Accepted Answer · 2020-12-06 23:12:51Z

2

You could use Unicode escapes in the x axis labels:

library(ggplot2)

ggplot(df, aes(theta, P_item_rasch)) +
  geom_line(aes(color = number)) +
  geom_segment(x = -1, xend = 1, y = 0.5, yend = 0.5, lty = 2) +

  geom_segment(x = -Inf, xend = -1, y = 0.5, yend = 0.5, lty = 2) +

  geom_segment(data = data.frame(theta = c(-1, 0, 1), P_item_rasch = rep(-Inf, 3)),
               aes(xend = theta, yend = 0.5), lty = 2) +
  scale_x_continuous(breaks = c(-6, -3, -1, 0, 1, 3, 6),
                     labels = c(-6, -3, "β\u1d62", 0, "β\u2c7c", 3, 6),
                     limits = c(-6, 6), name = "\u03b8") +

  scale_color_manual(values = RColorBrewer::brewer.pal(4, "Set1")[-1]) +
  theme_classic() +
  theme(legend.position = "none")

answered Dec 6, 2020 at 23:12

Allan Cameron

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3 Comments

firmo23 Over a year ago

just a comment. if I want to write βι=βj as a label how should I write it?

Allan Cameron Over a year ago

"β\u1d62 = β\u2c7c"

firmo23 Over a year ago

thank you again. is this Q possible? stackoverflow.com/questions/65184734/…

Duck · Accepted Answer · 2020-12-06 23:12:58Z

This can be useful. As I was not clear I used @AllanCameron solution for hint on placement of labels (many thanks):

library(ggplot2)
#Code
ggplot(df, aes(theta, P_item_rasch)) +
  geom_line(aes(color = number)) +
  #lims(x = c(-6, 6)) +
  # Line between curves
  geom_segment(x = -1, xend = 1, y = 0.5, yend = 0.5, lty = 2) +
  # Optional line on left
  geom_segment(x = -Inf, xend = -1, y = 0.5, yend = 0.5, lty = 2) +
  # Lower lines
  geom_segment(data = data.frame(theta = c(-1, 0, 1), P_item_rasch = rep(-Inf, 3)),
               aes(xend = theta, yend = 0.5), lty = 2) +
  # Upper lines
  #geom_segment(data = data.frame(theta = c(-1, 0, 1), P_item_rasch = rep(Inf, 3)),
  #            aes(xend = theta, yend = 0.5), lty = 2) +
  scale_color_manual(values = RColorBrewer::brewer.pal(4, "Set1")[-1]) +
  theme_classic() +
  theme(legend.position = "none")+
  scale_x_continuous(breaks = c(-6,-3,-1,0,1,3,6),limits = c(-6,6),
                     labels=c(-6,-3,expression(beta[i]),0,
                              expression(beta[j]),
                              3,6))+
  xlab(expression(theta))

Output:

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