修改箱线图的横坐标顺序

library(ggplot2)
#首先查看分组名字的levels
ggplot(mpg, aes(class, hwy)) + geom_boxplot()
#第一种使用，有低到高reorder
ggplot(mpg, aes(reorder(class, hwy), hwy)) + geom_boxplot()
#按照自己的想法顺序，画图。思路：重新添加一个子集，levels的顺序是我们想要的顺序
mpg$class1 <- factor(mpg$class,levels = c("compact","2seater","minivan","midsize","pickup","subcompact","suv"))
ggplot(mpg, aes(class1,hwy)) + geom_boxplot()