How can I make one plot from multiple variables in ggplot?

Question

I'm trying to make a pre-post change plot for three groups. In my data frame, pre data and post data are stored as two different variables like this.

pre_treat <- c(5,8,4,6,5,9,6,7,5)
post_treat <- c(2,2,4,10,9,11,4,5,3)
group <- c("A","A","A","B","B","B","C","C","C")
df <- data.frame(pre_treat = pre_treat,
                 post_treat = post_treat,
                 group = group)

> df
  pre_treat post_treat group
1         5          2     A
2         8          2     A
3         4          4     A
4         6         10     B
5         5          9     B
6         9         11     B
7         6          4     C
8         7          5     C
9         5          3     C

I want to plot mean values of pre_treat and post_treat for each group like the image below. I also want to plot them with group facets.

Answer 1

We could do it this way. First bring data in long format. Then calculate the mean for each group. Relevel with fct_relevel from forcats package and then plot with facet_wrap.

library(tidyverse)

df %>% 
  pivot_longer(-group) %>% 
  group_by(group, name) %>% 
  summarise(mean = mean(value)) %>% 
  mutate(name = fct_relevel(name, c("pre_treat", "post_treat"))) %>% 
  ggplot(aes(x=name, y=mean, group=1)) +
  geom_line()+
  facet_wrap(.~group)