How to change the order of the ggplot2 bar graph?

Question

I’m trying to change the order of the bars so that they appear in the same order as in fill (from top to bottom: a, b, c).

df3<-data.frame(Locus =rep(c("gene1","gene2"), 3),
            variable = c("a", "a", "b", "b", "c", "c"),
           value = c(-2.855996957,
           -2.859492255,
           -1.362416086,
           -5.36910352,
           1.493580871,
           -2.509611265))


ggplot(df3, aes(x = Locus, y = value, fill = variable, 
                order = as.numeric(variable))) +
  geom_col(position = "dodge") +
  scale_fill_manual(values=c("yellow", "red", "blue"))+
  labs(title = NULL,
       x = NULL,
       y = "",
       fill = "Tratamentos")+
  coord_flip()+
  theme_classic()

Fig. wrong:

Fig. desired:

Answer 1

Utilize position = position_dodge2(reverse = TRUE) within the function geom_col. The argument reverse = TRUE will reverse the bar order.

library(tidyverse)

df3 <-data.frame(Locus =rep(c("gene1","gene2"), 3),
                                variable = c("a", "a", "b", "b", "c", "c"),
                                value = c(-2.855996957,
                                                    -2.859492255,
                                                    -1.362416086,
                                                    -5.36910352,
                                                    1.493580871,
                                                    -2.509611265))


ggplot(df3, aes(x = Locus, y = value, fill = variable)) +
    geom_col(position = position_dodge2(reverse = TRUE, padding = 0)) +
    scale_fill_manual(values=c("yellow", "red", "blue"))+
    labs(title = NULL,
             x = NULL,
             y = "",
             fill = "Tratamentos")+
    coord_flip()+
    theme_classic()

^{Created on 2021-08-14 by the reprex package (v2.0.1)}

By default, the function position_dodge2 creates the bars with a spacing between them. So I put the argument padding = 0 so that the bars are glued to each other, as in the example provided.