在facet_网格中的条带外放置标签

left right facet_grid

作为一个可重复的例子,我想从这个例子开始

library(ggplot2)
    ggplot(mtcars, aes("", hp)) + 
      geom_boxplot(width=0.7, position=position_dodge(0.7)) + 
      theme_bw() +
      facet_grid(. ~ vs + am + carb,switch = 'both',labeller = label_both) +
      theme(panel.spacing=unit(0.2,"lines"),
            strip.background=element_rect(color="grey30", fill="grey90"),
            panel.border=element_rect(color="grey90"),
            axis.ticks.x=element_blank())+
            #strip.placement="outside") +
      labs(x="")

我寻找的情节;