Fix the middle category in ggplot

September 13, 2022

I have a bar graph with 3 groups, each with 3 fill categories. I want one category to always be the middle category on the bar graph.

dat <- data.frame(fill=c("Category A","Category B","Middle",
                      "Category A","Category C","Middle",
                      "Category B","Category C","Middle"),
                  x=c("A","A","A",
                      "B","B","B",
                      "C","C","C"),
                  y=c(.333,.333,.333,
                      .4,.4,.2,
                      .2,.4,.4))

ggplot(dat,aes(x=x,y=y,fill=fill))+
  geom_col()

I don’t think this is possible with traditional factor ordering because I can’t set the Middle category to be the middle of each combination:

dat$fill <- factor(dat$fill,levels=c("Category A","Middle","Category B","Category C"))

If I do this, Middle will fall between Category A and Category B, but not between Category B and Category C.

Is it possible to set one category to always be the middle category?

>Solution :

Here is a way to achieve this using dplyr:

library(dplyr)

dat %>% group_by(x) %>% 
  mutate(n = ifelse(fill=="Middle", 2, c(1,3)))%>%
  ggplot(.,aes(x=x,y=y,fill=fill, group=n))+
  geom_col()

The mutate will add the column you can use for your group ranked by order. The ifesle will give the row with "Middle" 2, and when false 1 and 3. In this case it is convenient as each group have 3 categories. This part will need change if you have more categories per group.

dat %>% group_by(x) %>% 
  mutate(n = ifelse(fill=="Middle", 2, c(1,3)))

# A tibble: 9 x 4
# Groups:   x [3]
  fill       x         y     n
  <chr>      <chr> <dbl> <dbl>
1 Category A A     0.333     1
2 Category B A     0.333     3
3 Middle     A     0.333     2
4 Category A B     0.4       1
5 Category C B     0.4       3
6 Middle     B     0.2       2
7 Category B C     0.2       1
8 Category C C     0.4       3
9 Middle     C     0.4       2