add significance asterisk manually in ggplot2

September 2, 2022

I have a time series DataFrame in R.
There are 4 groups and the data of each group (variable) is acquired at 3 different timepoints.

Group Timepoint Variable 
A        1 1.4705745 
B        1 4.6090900
C        1 2.2480962
D        1 1.6443650
E        1 4.4812444
A        2 0.8026552
B        2 4.7803944
C        2 1.3743527
D        2 4.0399467
E        2 3.5651057 
A        3 4.7275369
B        3 2.4491532
C        3 3.9508347
D        3 3.4278974
E        3 0.6917490

I made a line plot using the following code,

plot_data <- ggplot(data, aes(x = Timepoint, y = Variable, color = Group, group = Group))+geom_line()+
  scale_color_discrete("Group")+
  scale_y_continuous(limits = c(0, 6))
plot_data

but also want to add significant asterisk, for instance, like that.

Is there any way to add asterisk to the plot manually?

>Solution :

You can use annotate like this:

library(ggplot2)
plot_data <- ggplot(data, aes(x = Timepoint, y = Variable, color = Group, group = Group))+geom_line()+
  scale_color_discrete("Group")+
  scale_y_continuous(limits = c(0, 6)) +
  annotate('text', x = 1, y = c(5,5.2), label='"*"', parse=TRUE, color = c("pink", "yellow")) +
  annotate('text', x = 3, y = c(5,5.2), label='"*"', parse=TRUE, color = c("red", "green"))

plot_data

^{Created on 2022-09-02 with reprex v2.0.2}

To get the exact same colors as in standard ggplot you can check that by using hue_pal like this:

library(ggplot2)
library(scales)
show_col(hue_pal()(5))

plot_data <- ggplot(data, aes(x = Timepoint, y = Variable, color = Group, group = Group))+geom_line()+
  scale_color_discrete("Group")+
  scale_y_continuous(limits = c(0, 6)) +
  annotate('text', x = 1, y = c(5,5.2), label='"*"', parse=TRUE, color = c("#A3A500", "#E76BF3")) +
  annotate('text', x = 3, y = c(5,5.2), label='"*"', parse=TRUE, color = c("#F8766D", "#00BF7D"))

plot_data