如何为facet grid类型的ggplot绘制整体外框（排除图例与轴标签）

问题说明 #

我有一个facet grid类型的ggplot图表，需要给整个图表绘制一个外框：仅在x轴外侧、y轴外侧、分面标签上方和图表右侧各画一条线，形成包围所有分面的大框。使用plot.background = element_rect(...)只会给每个分面单独加框，无法实现整体外框的效果。

数据与原代码 #

library(tibble)
library(ggplot2)

df <- tibble::tribble(
  ~Habitat_Year_Cohort, ~Month,         ~Location,             ~avg,               ~sd,  ~Year,                   ~Habitat,     ~Habitat_Year_Cohort_2,
  "Seagrass (hard-bottom) 2005",  "Dec",       "Birch Bay", 1.22657142857143, 0.461769010488598, "2005",   "Seagrass (hard-bottom)",   "Seagrass (hard-bottom)",
  "Seagrass (soft-bottom) 2005",  "Dec",     "Padilla Bay",            1.405, 0.518022200296474, "2005",   "Seagrass (soft-bottom)",   "Seagrass (soft-bottom)",
  "Mussel (mixed substrate) 2005",  "Dec",       "Penn Cove",         1.803125, 0.523759878870352, "2005", "Mussel (mixed substrate)", "Mussel (mixed substrate)",
  "After Hatch Year Male 2021",  "Dec",      "Adult Male", 1.29973684210526, 0.492007434689227, "2021", "Mussel (mixed substrate)",    "After Hatch Year Male",
  "After Hatch Year Female 2021",  "Dec",    "Adult Female", 1.24852173913043, 0.405312547140556, "2021", "Mussel (mixed substrate)",  "After Hatch Year Female",
  "Hatch Year Male 2021",  "Dec",   "Juvenile Male",             1.09, 0.270336826940023, "2021", "Mussel (mixed substrate)",          "Hatch Year Male",
  "Hatch Year Female 2021",  "Dec", "Juvenile Female",            1.233, 0.298399061660723, "2021", "Mussel (mixed substrate)",        "Hatch Year Female",
  "Seagrass (hard-bottom) 2006",  "Mar",       "Birch Bay", 1.06914285714286, 0.334118456325212, "2006",   "Seagrass (hard-bottom)",   "Seagrass (hard-bottom)",
  "Seagrass (soft-bottom) 2006",  "Mar",     "Padilla Bay",         1.140125, 0.248227682063982, "2006",   "Seagrass (soft-bottom)",   "Seagrass (soft-bottom)",
  "Mussel (mixed substrate) 2006",  "Mar",       "Penn Cove",           0.8865, 0.181639360114643, "2006", "Mussel (mixed substrate)", "Mussel (mixed substrate)"
)

ggplot(data = df, aes(x = Month, y = avg, col = Habitat, group = Habitat_Year_Cohort)) +
  geom_errorbar(aes(ymin = avg - sd, ymax = avg + sd),
    width = 0.3,
    position = position_dodge(width = 2)
  ) +
  geom_point(position = position_dodge(width = 2), show.legend = F) +
  facet_grid(~Year, scales = "free") +
  geom_text(aes(label = Location), show.legend = F, angle = 90, position = position_dodge(width = 2), vjust = -0.5, col = "black") +
  theme(
    legend.position = "bottom",
    legend.key = element_blank(),
    panel.background = element_blank(),
    panel.border = element_blank(),
    strip.background = element_blank(),
    plot.background = element_blank(),
    panel.grid = element_blank()
  ) +
  scale_color_manual(
    values = c(
      "Seagrass (hard-bottom)" = "grey75",
      "Seagrass (soft-bottom)" = "grey50",
      "Mussel (mixed substrate)" = "grey1"
    )
  ) +
  annotate("segment", x = Inf, xend = -Inf, y = Inf, yend = Inf, color = "black", lwd = 1)

解决方案 #

方法1：用annotate添加四条线段 #

通过annotate()分别绘制外框的四条边，利用Inf和-Inf自动适配整个图表的边界范围，同时调整绘图边距避免外框与轴标签重叠：

ggplot(data = df, aes(x = Month, y = avg, col = Habitat, group = Habitat_Year_Cohort)) +
  geom_errorbar(aes(ymin = avg - sd, ymax = avg + sd),
                width = 0.3,
                position = position_dodge(width = 2)
  ) +
  geom_point(position = position_dodge(width = 2), show.legend = FALSE) +
  facet_grid(~Year, scales = "free") +
  geom_text(aes(label = Location), show.legend = FALSE, angle = 90, position = position_dodge(width = 2), vjust = -0.5, col = "black") +
  theme(
    legend.position = "bottom",
    legend.key = element_blank(),
    panel.background = element_blank(),
    panel.border = element_blank(),
    strip.background = element_blank(),
    plot.background = element_blank(),
    panel.grid = element_blank(),
    # 调整边距，避免外框遮挡轴标签
    plot.margin = margin(10, 10, 10, 10)
  ) +
  scale_color_manual(
    values = c(
      "Seagrass (hard-bottom)" = "grey75",
      "Seagrass (soft-bottom)" = "grey50",
      "Mussel (mixed substrate)" = "grey1"
    )
  ) +
  # 顶部外框线（分面标签上方）
  annotate("segment", x = -Inf, xend = Inf, y = Inf, yend = Inf, color = "black", lwd = 1) +
  # 右侧外框线
  annotate("segment", x = Inf, xend = Inf, y = -Inf, yend = Inf, color = "black", lwd = 1) +
  # 底部外框线（x轴外侧）
  annotate("segment", x = -Inf, xend = Inf, y = -Inf, yend = -Inf, color = "black", lwd = 1) +
  # 左侧外框线（y轴外侧）
  annotate("segment", x = -Inf, xend = -Inf, y = -Inf, yend = Inf, color = "black", lwd = 1)

方法2：用patchwork包添加整体边框 #

如果安装了patchwork包，可以将整个图表作为一个对象，通过plot_annotation()添加整体边框：

library(patchwork)

# 先绘制原图表
p <- ggplot(data = df, aes(x = Month, y = avg, col = Habitat, group = Habitat_Year_Cohort)) +
  geom_errorbar(aes(ymin = avg - sd, ymax = avg + sd),
                width = 0.3,
                position = position_dodge(width = 2)
  ) +
  geom_point(position = position_dodge(width = 2), show.legend = FALSE) +
  facet_grid(~Year, scales = "free") +
  geom_text(aes(label = Location), show.legend = FALSE, angle = 90, position = position_dodge(width = 2), vjust = -0.5, col = "black") +
  theme(
    legend.position = "bottom",
    legend.key = element_blank(),
    panel.background = element_blank(),
    panel.border = element_blank(),
    strip.background = element_blank(),
    plot.background = element_blank(),
    panel.grid = element_blank()
  ) +
  scale_color_manual(
    values = c(
      "Seagrass (hard-bottom)" = "grey75",
      "Seagrass (soft-bottom)" = "grey50",
      "Mussel (mixed substrate)" = "grey1"
    )
  )

# 添加整体外框
p + plot_annotation(theme = theme(plot.background = element_rect(color = "black", fill = NA, size = 1)))

说明 #

• 方法1无需额外安装包，直接通过ggplot原生函数实现，适配scales="free"的分面场景；
• 方法2借助patchwork的布局能力，代码更简洁，但需要安装patchwork包。

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