R 堆叠条形图中的顺序

我有下面的数据框，没什么好想象的

df_bar<-data.frame(capacity = c(no[2],no[1]-no[2],max(df_l$load)-no[1]), type = c("Nuclear","Coal","Gas"),a = c("Optimal","Optimal","Optimal"))

我怎样才能修改代码，使其符合我需要的顺序？

1.创建一个

ggplot(df_bar, aes(y=capacity, x=a, fill=type)) + 
geom_bar(position="stack", stat="identity")

df_bar<-data.frame(capacity = c(2,2,2),
                   type = c("Nuclear","Coal","Gas"),
                   a = c("Optimal","Optimal","Optimal"))

library(ggplot2)
ggplot(df_bar, aes(y=capacity, x=a, fill=factor(type, levels=c( "Coal", "Gas", "Nuclear")))) + 
  geom_bar(position="stack", stat="identity") +
  labs(fill="type")