如何在ggplot2的不同分面中添加对应水平线？

问题场景 #

你用ggplot2绘制了按NutrientType分面的柱状图，想要给TN和TP两个分面分别添加对应指导值的水平线（TN对应900，TP对应250），但当前用geom_hline的方法会把两条水平线同时显示在每个分面中，无法实现分面对应。

数据与现有代码 #

原始数据 #

BARR1 <- tibble::tribble(
  ~Trip,   ~Depth,       ~NutrientType, ~ugL,
  "Before", "Surface",     "TN",            1111,
  "Before", "Surface",     "TP",             162,
  "Before", "Near bottom", "TN",            1428,
  "Before", "Near bottom", "TP",             191,
  "After",  "Surface",     "TN",             869,
  "After",  "Surface",     "TP",             756,
  "After",  "Near bottom", "TN",             816,
  "After",  "Near bottom", "TP",             487
)

现有绘图代码 #

colours <- c("#your_color1", "#your_color2") # 替换为你实际使用的颜色
BARR1plot <- ggplot(
  BARR1,
  aes(
    x = Depth,
    y = ugL,
    fill = Trip
  )
) +
  geom_bar(colour = "black", stat = "identity", position = "dodge") +
  scale_fill_manual(values = colours) +
  theme_classic() +
  theme(text = element_text(size = 18)) +
  theme(axis.text.x = element_text(angle = 90, hjust = 1, vjust = 0.5, size = 12)) +
  theme(axis.title.y = element_text(size = 16)) +
  theme(legend.title = element_blank()) +
  ylab("Concentration (µg/L)") +
  xlab("") +
  facet_grid(~NutrientType, scales = "fixed", switch = "x") +
  labs(title = "BARR1") +
  theme(plot.title = element_text(hjust = 0)) +
  theme(panel.spacing = unit(0, "mm"))

解决方案 #

问题出在水平线数据框的分组列名与分面变量不匹配，导致ggplot无法关联水平线和对应分面。只需调整水平线数据框的列名，让它和分面变量NutrientType一致即可：

修正后的水平线代码 #

# 创建匹配分面变量的水平线数据框
data_hline <- data.frame(
  NutrientType = c("TN", "TP"), # 列名与分面变量完全一致
  hline = c(900, 250)           # 对应各自的指导值
)

# 添加水平线到绘图
BARR1plot +
  geom_hline(
    data = data_hline,
    aes(yintercept = hline)
  )

可选：自定义水平线样式 #

如果需要突出显示水平线，可以添加颜色、线型等样式：

BARR1plot +
  geom_hline(
    data = data_hline,
    aes(yintercept = hline),
    color = "darkred",    # 设置颜色
    linetype = "dashed",  # 设置线型
    size = 1              # 设置线宽
  )

修改后，ggplot会自动将TN对应的水平线显示在TN分面，TP对应的水平线显示在TP分面，不会再出现交叉显示的问题。

内容的提问来源于stack exchange，提问作者Lindsey