如何在ggplot坐标轴上高亮特定因子水平（如指定车型）？

方法1：高亮X轴车型名称（加粗彩色） #

核心思路是先标记需要高亮的车型，再通过scale_x_discrete自定义轴文本样式：

data(mtcars)
library(tidyverse)

# 标记需要高亮的车型
mtcars_highlight <- mtcars %>%
  mutate(
    cars = rownames(mtcars),
    is_highlight = cars %in% c("Fiat 128", "Volvo 142E")
  )

ggplot(mtcars_highlight, aes(x = cars, y = mpg)) +
  geom_point() +
  # 自定义X轴文本：高亮车型用红色加粗，其余默认
  scale_x_discrete(labels = function(x) {
      ifelse(x %in% c("Fiat 128", "Volvo 142E"), 
             paste0("**", x, "**"),
             x)
    }) +
  theme(
    axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1,
                               color = ifelse(mtcars_highlight$is_highlight, "red", "black"),
                               face = ifelse(mtcars_highlight$is_highlight, "bold", "plain"))
  )

方法2：给目标车型的散点添加圆圈高亮 #

在原有散点上叠加空心大圆圈，直接突出指定车型：

data(mtcars)
library(tidyverse)

mtcars %>%
  mutate(cars = rownames(mtcars)) %>%
  ggplot(aes(x = cars, y = mpg)) +
  # 先绘制所有普通散点
  geom_point(color = "black", size = 2) +
  # 叠加高亮车型的空心圆圈
  geom_point(
    data = . %>% filter(cars %in% c("Fiat 128", "Volvo 142E")),
    aes(x = cars, y = mpg),
    color = "red",
    size = 5,
    shape = 21,  # 空心圆圈形状
    fill = NA,   # 内部透明
    stroke = 2   # 圆圈线条粗细
  ) +
  theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1))

如果需要更规整的圆形高亮，可使用ggforce包：

data(mtcars)
library(tidyverse)
library(ggforce)

mtcars %>%
  mutate(cars = rownames(mtcars)) %>%
  ggplot(aes(x = cars, y = mpg)) +
  geom_point() +
  # 给目标车型添加精准圆形高亮
  geom_circle(
    data = . %>% filter(cars %in% c("Fiat 128", "Volvo 142E")),
    aes(x0 = as.numeric(factor(cars)), y0 = mpg, r = 0.3),
    color = "red",
    size = 1.5,
    fill = NA
  ) +
  theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1))

内容的提问来源于stack exchange，提问作者Marco