ggplot2饼图标题置底问题：vjust参数无效，求规范解决方案

问题根源：使用coord_polar()配合theme_void()后，绘图的坐标系统和边距逻辑发生改变，vjust的参考基准不再是常规的绘图区域，导致小数值无法将标题推至底部。以下是两种规范的解决方式：

方式一：改用plot.caption放置标题（推荐） #

theme_void()默认会将caption置于底部，只需调整样式和边距即可：

library(tidyverse)    
pie_tbl <- tibble(
  surface = factor(c("Dry","Wet","Snow/Ice","Other"),levels=c("Dry","Wet","Snow/Ice","Other")),
  percentage = c(78,14.8,6.9,0.2)
)

pie_tbl %>% ggplot(aes(x="",y=percentage,fill=surface)) + 
  geom_col(color='black',width=1) + 
  coord_polar("y",start=pi/2) +
  labs(caption="Surface Condition during collisions") +  # 用caption替代title
  theme_void() +
  theme(
    plot.caption = element_text(family='serif',face='bold',size=18,hjust=0.5),
    plot.margin = margin(b = 20)  # 给底部预留空间，避免标题被裁切
  ) +
  scale_fill_manual(
    values=c(
      "Dry" = "#27B7CE",
      "Wet" = "#FB6476",
      "Snow/Ice" = "#47B972",
      "Other" = "#FEB20E"
    )
  )

方式二：保留title，用margin参数控制位置 #

如果必须使用title，可以通过element_text()的margin参数将标题向下推送，同时调整绘图边距：

library(tidyverse)    
pie_tbl <- tibble(
  surface = factor(c("Dry","Wet","Snow/Ice","Other"),levels=c("Dry","Wet","Snow/Ice","Other")),
  percentage = c(78,14.8,6.9,0.2)
)

pie_tbl %>% ggplot(aes(x="",y=percentage,fill=surface)) + 
  geom_col(color='black',width=1) + 
  coord_polar("y",start=pi/2) +
  labs(title="Surface Condition during collisions") +
  theme_void() +
  theme(
    plot.title = element_text(
      family='serif',face='bold',size=18,hjust=0.5,
      margin = margin(b = 100)  # 添加底部外边距，将标题推至下方
    ),
    plot.margin = margin(b = 20)  # 确保底部有足够空间显示标题
  ) +
  scale_fill_manual(
    values=c(
      "Dry" = "#27B7CE",
      "Wet" = "#FB6476",
      "Snow/Ice" = "#47B972",
      "Other" = "#FEB20E"
    )
  )

内容的提问来源于stack exchange，提问作者alex94