引言

ggplot2 生成的 facet 标签顺序可能无法满足特定的需求，而标签格式的限制也可能使得科学符号和公式难以正确显示。

怎么定制 facet 标签？包括设置标签的显示顺序和添加公式等格式调整？怎么拼图？记录之。

包：各种工具

library(ggplot2)   # 绘图
library(ggtext)    # 文本格式、颜色、大小等
library(ggpmisc)   # 添加 P 值、R 方、回归公式等
library(patchwork) # 拼图
library(svglite)   # 保存 SVG 矢量图以便后续在 PPT 里编辑

主题：绘图风格

font <- "DejaVu Sans"

theme_set(
  theme_bw(
    base_family = font,
    base_size = 16,
    base_rect_size = 0.8,
    base_line_size = 0.4
  ) +
    theme(
      panel.grid = element_blank(),
      strip.background = element_blank(),
      strip.clip = "off",
      axis.ticks.length = unit(4, "pt")
    )
)

初始底图

子图：一

colnames(mpg)

 [1] "manufacturer" "model"        "displ"        "year"         "cyl"         
 [6] "trans"        "drv"          "cty"          "hwy"          "fl"          
[11] "class"       

p1 <- ggplot(
  data = mpg,
  aes(x = displ, y = hwy)
) +
  geom_point(color = "grey50") +
  stat_smooth(method = lm, formula = y ~ x, color = "grey0", fullrange = TRUE, se = FALSE) +
  stat_poly_eq(use_label("r2"), method = lm, formula = y ~ x, family = font) +
  facet_grid("A: hwy ~ displ" ~ class, switch = "x") +
  labs(x = NULL, y = expression(R[s]~"("*mu*l~CO[2]~h^-1*")")) +
  theme(
    panel.spacing = unit(-1, "pt"),
    strip.background.y = element_rect(fill = "grey90", linewidth = 0.4),
    strip.placement = "outside"
  )
p1

子图：二

p2 <- ggplot(
  data = mpg,
  aes(x = displ, y = cty)
) +
  geom_point(color = "grey50") +
  stat_smooth(method = lm, formula = y ~ x, color = "grey0", fullrange = TRUE, se = FALSE) +
  stat_poly_eq(use_label("r2"), method = lm, formula = y ~ x, family = font) +
  facet_grid("B: cty ~ displ" ~ class, switch = "x") +
  labs(x = NULL, y = expression(R[s]~"("*mu*l~CO[2]~h^-1*")")) +
  theme(
    panel.spacing = unit(-1, "pt"),
    strip.background.y = element_rect(fill = "grey90", linewidth = 0.4),
    strip.placement = "outside"
  )
p2

拼图

fig <- p1 + p2 + plot_layout(ncol = 1)
fig

进一步修图

facet 标签：顺序

• 设置 facet 变量设置为指定 levels 的 factor；

• levels 就是 facet 的顺序。

unique(mpg$class)

[1] "compact"    "midsize"    "suv"        "2seater"    "minivan"   
[6] "pickup"     "subcompact"

mpg$class <- factor(
  mpg$class,
  levels = c(
    "2seater",
    "subcompact",
    "compact",
    "midsize",
    "suv",
    "minivan",
    "pickup"
  )
)

levels(mpg$class)

[1] "2seater"    "subcompact" "compact"    "midsize"    "suv"       
[6] "minivan"    "pickup"    

facet 标签：上下标

• 注：使用 atop 换行。

unique(mpg$class)

[1] compact    midsize    suv        2seater    minivan    pickup     subcompact
Levels: 2seater subcompact compact midsize suv minivan pickup

class <- c(
  "2seater",
    "subcompact",
    "compact",
    "midsize",
    "suv",
    "minivan",
    "pickup"
)

class.expr <- c(
  "2~seater", # 数字和文字之间需 ~ 或 * 连接
  "atop(subcompact,(NH[4]^'+'))",
  "atop(compact,(g~kg^-1))",
  "midsize",
  "suv",
  "minivan~(g~kg^-1)",
  "atop(pickup,(NO[3]^'-'))"
)

class2 <- setNames(class.expr, class)
mpg$class2 <- class2[mpg$class]
mpg$class2 <- factor(mpg$class2, levels = class.expr)

levels(mpg$class2)

[1] "2~seater"                     "atop(subcompact,(NH[4]^'+'))"
[3] "atop(compact,(g~kg^-1))"      "midsize"                     
[5] "suv"                          "minivan~(g~kg^-1)"           
[7] "atop(pickup,(NO[3]^'-'))"    

ggplot(
  data = mpg,
  aes(x = displ, y = hwy)
) +
  geom_point(color = "grey50") +
  stat_smooth(method = lm, formula = y ~ x, color = "grey0", fullrange = TRUE, se = FALSE) +
  stat_poly_eq(use_label("r2"), method = lm, formula = y ~ x, family = font) +
  facet_grid(
    "L:~20~cm" ~ class2,
    switch = "x",
    labeller = label_parsed
  ) +
  labs(x = NULL, y = expression(R[s]~"("*mu*l~CO[2]~h^-1*")")) +
  theme(
    panel.spacing = unit(-1, "pt"),
    strip.background.y = element_rect(fill = "grey90", linewidth = 0.4),
    strip.placement = "outside"
  )