更改ggplot facet_wrap的facet标题格式

使用以下代码获取由以下数据生成的绘图：

library(dplyr)

dd<-data.frame(matrix(0,36,1))
dd$drug<-c("x","x","x","x","x","x","x","x","x","y","y","y","y","y","y","y","y","y","z","z","z","z","z","z","z","z","z","w","w","w","w","w","w","w","w","w")
dd$prev<-c(10,11,12,11,12,13,12,13,14,10,11,12,11,12,13,12,13,14,10,11,12,11,12,13,12,13,14,10,11,12,11,12,13,12,13,14)
dd$country<-c("a","a","a","b","b","b","c","c","c","a","a","a","b","b","b","c","c","c","a","a","a","b","b","b","c","c","c","a","a","a","b","b","b","c","c","c")
dd$year<-c(15,16,17,15,16,17,15,16,17,15,16,17,15,16,17,15,16,17,15,16,17,15,16,17,15,16,17,15,16,17,15,16,17,15,16,17)
dd<-dd[,-1]

dd <- dd %>%
  group_by(drug) %>%
  arrange(country) %>%
  ungroup()

ggplot(dd,aes(x=factor(year),y=prev,fill = factor(country,levels=c("a","b","c")),pos="dodge"))+
  geom_col(col = "black",size=0.25,pos=position_dodge(0.9)) +
  facet_wrap(~factor(drug, levels(factor(dd$drug))[c(2,1,3,4)]),strip.position="bottom",as.table=T,scales="free_x")+
  scale_fill_manual(values = c("a" = "white",
                           "b" = "lightskyblue2",
                           "c" = "lightpink"),
                guide = F)+
  scale_x_discrete(name="Year",labels=c(2015,2016,2017))+
  scale_y_continuous(name="Prevalence (%)")+
  theme_classic()

库（dplyr）
dd检查
？主题
并阅读
条带的所有选项。*
。相关的是位置和背景。对于间距，有
面板。间距

ggplot(mtcars, aes(factor(am))) +
  geom_bar() +
  facet_wrap(~cyl, strip.position = "bottom", nrow = 2, scales = "free") +
  theme_classic() +
  theme(strip.placement = "outside",
        strip.background = element_blank(),
        panel.spacing.y = unit(2, "lines"))