R中使用facet_grid()仅保留最外层边框的实现需求

当使用facet_grid(~Season, switch='x')将分面标签置于绘图底部时，常规的panel.border设置会为每个分面单独添加边框，导致出现多余的水平分隔线。以下提供两种可靠方案，仅保留整个绘图区域的最外层边框：

方案一：使用patchwork包添加外层边框 #

借助patchwork可以轻松为整个绘图包裹统一边框，步骤如下：

library(ggplot2)
library(patchwork)

# 数据准备
df <- structure(list(Lake = c("Lake_x", "Lake_x", "Lake_x", "Lake_x"
), Site = c("Lake_x-1", "Lake_x-2", "Lake_x-3", "Lake_x-4"), 
Season = structure(1:4, levels = c("Season1", "Season2", 
                                   "Season3", "Season4"), class = "factor"), Nitrogen = c(450L, 
                                                                                          390L, 930L, 950L)), row.names = c(NA, -4L), class = "data.frame")

df$Season <- factor(df$Season, levels=c("Season1", "Season2", "Season3", "Season4"))

# 绘制基础图形（移除原panel.border设置）
p <- ggplot(df, aes(fill = Site, y = Nitrogen , x = Site)) +
  geom_bar(stat="identity", position = position_dodge(width = 6), width = 0.4) +
  scale_fill_manual("Site", values = rep("black", 4)) +
  theme_classic() +
  theme(text = element_text(size = 8, family = "sans")) + 
  labs(title = 'Area 8', x = "", y = expression(paste("Total Nitrogen (µg/L)"))) +
  scale_y_continuous(expand = c(0,0)) +  
  scale_x_discrete(NULL, expand = c(0.2, 0.2)) +
  theme(legend.position ="none") +
  facet_grid(~Season, scales = "free_x", switch = 'x') +
  theme(panel.background = element_blank(),
        panel.spacing = unit(0, "line"), 
        strip.background = element_blank(),
        strip.placement = "outside") + 
  theme(plot.title.position = 'plot', 
        plot.title = element_text(hjust = 0.5)) + 
  theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust=1)) 

# 为整个图形添加外层边框
wrap_plots(p) +
  plot_annotation(
    theme = theme(
      plot.background = element_rect(colour = "black", fill = NA, size = 1),
      plot.margin = margin(10, 10, 10, 10) # 调整边距确保边框完整显示
    )
  )

方案二：纯ggplot2实现（无需额外包） #

通过annotation_custom绘制覆盖整个绘图区域的矩形边框，同时关闭坐标裁剪以确保边框完整显示：

library(ggplot2)

# 数据准备
df <- structure(list(Lake = c("Lake_x", "Lake_x", "Lake_x", "Lake_x"
), Site = c("Lake_x-1", "Lake_x-2", "Lake_x-3", "Lake_x-4"), 
Season = structure(1:4, levels = c("Season1", "Season2", 
                                   "Season3", "Season4"), class = "factor"), Nitrogen = c(450L, 
                                                                                          390L, 930L, 950L)), row.names = c(NA, -4L), class = "data.frame")

df$Season <- factor(df$Season, levels=c("Season1", "Season2", "Season3", "Season4"))

ggplot(df, aes(fill = Site, y = Nitrogen , x = Site)) +
  geom_bar(stat="identity", position = position_dodge(width = 6), width = 0.4) +
  scale_fill_manual("Site", values = rep("black", 4)) +
  theme_classic() +
  theme(text = element_text(size = 8, family = "sans")) + 
  labs(title = 'Area 8', x = "", y = expression(paste("Total Nitrogen (µg/L)"))) +
  scale_y_continuous(expand = c(0,0)) +  
  scale_x_discrete(NULL, expand = c(0.2, 0.2)) +
  theme(legend.position ="none") +
  facet_grid(~Season, scales = "free_x", switch = 'x') +
  theme(panel.background = element_blank(),
        panel.spacing = unit(0, "line"), 
        strip.background = element_blank(),
        strip.placement = "outside",
        panel.border = element_blank()) + # 移除分面单独边框
  theme(plot.title.position = 'plot', 
        plot.title = element_text(hjust = 0.5)) + 
  theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust=1)) +
  # 添加外层边框
  annotation_custom(
    grob = rectGrob(gp = gpar(col = "black", lwd = 1, fill = NA)),
    xmin = -Inf, xmax = Inf, ymin = -Inf, ymax = Inf
  ) +
  coord_cartesian(clip = "off") + # 关闭裁剪让边框完整显示
  theme(plot.margin = margin(10, 10, 10, 10)) # 调整边距避免边框被截断

方案说明 #

• 方案一依赖patchwork包，代码简洁直观，适合需要多图组合的场景；
• 方案二无需额外依赖，纯ggplot2实现，适合环境受限的场景，需注意coord_cartesian(clip = "off")和边距的设置，确保边框完整显示。

内容的提问来源于stack exchange，提问作者confusedindividual