方法 1：效果不错

缺点是，需要手动调整图左侧边缘 margin 的宽度，手动设置标签 label 和线段 segment 的 X 坐标，

• 反复调试 X 坐标，直到合适为止。

# FONT <- "Myriad Pro"
FONT <- "Open Sans"

p0 <- ggplot(
  data = mpg2,
  aes(x = hwy, y = model)
) +
  geom_point(size = 1) +
  
  geom_text(
    data = mpg2 |> 
      slice(1, .by = c(class, model)) |>
      summarise(y.pos = n() / 2 + 0.5, .by = class) |> 
      filter(class %in% c("minivan-MPV", "subcompact-B-segment")),
    aes(x = -23, y = y.pos, label = class),
    size = 3.5,
    color = "dodgerblue2",
    hjust = 0.5,
    angle = 90,
    family = FONT
  ) +
  geom_segment(
    data = mpg2 |> 
      slice(1, .by = c(class, model)) |> 
      summarise(n.model = n(), .by = class) |> 
      mutate(
        y.pos.max = n.model + 1,
        y.pos.min = 0,
        .by = class
      ) |> 
      filter(class %in% c("minivan-MPV", "subcompact-B-segment")),
    aes(x = -21, xend = -21, y = y.pos.min, yend = y.pos.max),
    size = 0.4,
    color = "dodgerblue2"
  ) +
  
  geom_text(
    data = mpg2 |> 
      slice(1, .by = c(class, model)) |>
      summarise(y.pos = n() / 2 + 0.5, .by = class) |> 
      filter(!class %in% c("minivan-MPV", "subcompact-B-segment")),
    aes(x = -19, y = y.pos, label = class),
    size = 3.5,
    color = "green4",
    hjust = 0.5,
    angle = 90,
    family = FONT
  ) +
  geom_segment(
    data = mpg2 |> 
      slice(1, .by = c(class, model)) |> 
      summarise(n.model = n(), .by = class) |> 
      mutate(
        y.pos.max = n.model + 1,
        y.pos.min = 0,
        .by = class
      ) |> 
      filter(!class %in% c("minivan-MPV", "subcompact-B-segment")),
    aes(x = -17, xend = -17, y = y.pos.min, yend = y.pos.max),
    size = 0.4,
    color = "green4"
  ) +
  
  scale_x_continuous(
    expand = expansion(mult = c(0, 0.06))
  ) +
  facet_grid(
    class ~ .,
    scales = "free_y",
    space = "free_y",
    switch = "y"
  ) +
  labs(
    x = "Highway miles per gallon",
    y = NULL
  ) +
  coord_cartesian(
    xlim = c(10, 50),
    clip = "off"
  ) +
  theme_classic(
    base_family = FONT,
    base_line_size = 0.3
  ) +
  theme(
    plot.margin = margin(20, 5, 5, 40),
    strip.background = element_blank(),
    strip.text = element_blank(),
    strip.clip = "off",
    strip.placement = "outside",
    axis.title = element_text(size = 10),
    axis.text = element_text(size = 10, color = "black"),
  )
p0

# ggview::ggview(p0, width = 4.9, height = 7, dpi = 300)

ggsave(
  "as-two-rows-facet-perfect.png",
  p0,
  width = 4.9,
  height = 7,
  dpi = 600
)

方法 2：通过两部分拼图实现，更为灵活

mpg2 <- mpg
mpg2$class <- ifelse(mpg2$class == "minivan", "minivan-MPV", mpg2$class)
mpg2$class <- ifelse(mpg2$class == "subcompact", "subcompact-B-segment", mpg2$class)

theme_set(
  theme_classic(
    base_family = FONT,
    base_rect_size = 0.3,
    base_line_size = 0.3
  ) +
    theme(
      strip.background = element_blank(),
      strip.clip = "off",
      strip.placement = "outside",
      axis.title = element_text(size = 10),
      axis.text = element_text(size = 10, color = "black"),
    )
)

# Prepare p2a
p2a <- ggplot(
  data = mpg2,
  aes(x = hwy, y = model)
) +
  geom_point(size = 1) +
  
  scale_x_continuous(
    expand = expansion(mult = c(0, 0.06))
  ) +
  scale_y_discrete(
    expand = expansion(add = c(0.5, 0.5))
  ) +
  facet_grid(
    class ~ .,
    scales = "free_y",
    space = "free_y",
    switch = "y"
  ) +
  labs(
    x = "Highway miles per gallon",
    y = NULL
  ) +
  coord_cartesian(
    xlim = c(10, 50),
    clip = "off"
  ) +
  theme(
    strip.text = element_blank()
  )
# ggview::ggview(
#   p2a,
#   width = 3,
#   height = 7,
#   dpi = 900
# )

# Prepare p2b
p2b <- ggplot(
  data = mpg2,
  aes(x = hwy, y = model)
) +
  geom_blank() +
  
  geom_text(
    data = mpg2 |> 
      slice(1, .by = c(class, model)) |>
      summarise(y.pos = n() / 2 + 0.5, .by = class) |> 
      filter(
        class %in% c("minivan-MPV", "subcompact-B-segment")
      ),
    aes(x = 17, y = y.pos, label = class),
    size = 3.5,
    color = "dodgerblue2",
    hjust = 0.5,
    angle = 90,
    family = FONT,
    inherit.aes = FALSE
  ) +
  geom_segment(
    data = mpg2 |> 
      slice(1, .by = c(class, model)) |>
      summarise(n.model = n(), .by = class) |> 
      mutate(
        y.pos.max = n.model + 0.5,
        y.pos.min = 0.5,
        .by = class
      ) |> 
      filter(
        class %in% c("minivan-MPV", "subcompact-B-segment")
      ),
    aes(x = 20, xend = 20, y = y.pos.min, yend = y.pos.max),
    size = 0.4,
    color = "dodgerblue2"
  ) +
  
  geom_text(
    data = mpg2 |> 
      slice(1, .by = c(class, model)) |>
      summarise(y.pos = n() / 2 + 0.5, .by = class) |> 
      filter(
        !class %in% c("minivan-MPV", "subcompact-B-segment")
      ),
    aes(x = 24, y = y.pos, label = class),
    size = 3.5,
    color = "green4",
    hjust = 0.5,
    angle = 90,
    family = FONT,
    inherit.aes = FALSE
  ) +
  geom_segment(
    data = mpg2 |> 
      slice(1, .by = c(class, model)) |>
      summarise(n.model = n(), .by = class) |> 
      mutate(
        y.pos.max = n.model + 0.5,
        y.pos.min = 0.5,
        .by = class
      ) |> 
      filter(
        !class %in% c("minivan-MPV", "subcompact-B-segment")
      ),
    aes(x = 27, xend = 27, y = y.pos.min, yend = y.pos.max),
    size = 0.4,
    color = "green4"
  ) +
  
  scale_x_continuous(
    expand = expansion(mult = c(0, 0.06))
  ) +
  scale_y_discrete(
    expand = expansion(add = c(0.5, 0.5))
  ) +
  facet_grid(
    class ~ .,
    scales = "free_y",
    space = "free_y",
    switch = "y"
  ) +
  labs(
    x = NULL,
    y = NULL
  ) +
  coord_cartesian(
    xlim = c(17, 27),
    clip = "off"
  ) +
  theme(
    panel.background = element_blank(),
    strip.text = element_blank(),
    axis.text = element_text(size = 0),
    axis.line = element_blank(),
    axis.ticks = element_blank()
  )

# 拼图
(p2 <- p2b + p2a + plot_layout(widths = c(1, 5)))

# ggview::ggview(p2, width = 4.9, height = 7.5, dpi = 300)

ggsave(
  "as-two-rows-plot-combine.png",
  p2,
  width = 4.9,
  height = 7.5,
  dpi = 600
)

方法 3：不完美，但富文本格式，可设置字体颜色

• 小技巧：#ffffff00 是透明色。

mpg2 <- mpg
mpg2$class <- ifelse(mpg2$class == "minivan", "minivan-MPV", mpg2$class)
mpg2$class <- ifelse(mpg2$class == "subcompact", "subcompact-B-segment", mpg2$class)

glue.color.string <- function(clr, str, cex = 10) {
  glue::glue(
    "<span style = 'color:{color}; font-size:{size}pt'>{text}</span>",
    color = clr,
    size = cex,
    text = str
  )
}
glue.color.string("red", "minivan-MPV")

<span style = 'color:red; font-size:10pt'>minivan-MPV</span>

class.levels <- ifelse(
  mpg2$class %in% c("minivan-MPV", "subcompact-B-segment"),
  glue.color.string("#ffffff00", mpg2$class),
  glue.color.string("black", mpg2$class)
) |>
  unique()
print(class.levels)

[1] "<span style = 'color:black; font-size:10pt'>compact</span>"                 
[2] "<span style = 'color:black; font-size:10pt'>midsize</span>"                 
[3] "<span style = 'color:black; font-size:10pt'>suv</span>"                     
[4] "<span style = 'color:black; font-size:10pt'>2seater</span>"                 
[5] "<span style = 'color:#ffffff00; font-size:10pt'>minivan-MPV</span>"         
[6] "<span style = 'color:black; font-size:10pt'>pickup</span>"                  
[7] "<span style = 'color:#ffffff00; font-size:10pt'>subcompact-B-segment</span>"

• 优点是，富文本格式可设置各种颜色。

theme_set(
  theme_classic(
    base_family = FONT,
    base_line_size = 0.3
  ) +
    theme(
      plot.margin = margin(30, 5, 5, 5),
      strip.background = element_blank(),
      strip.clip = "off",
      strip.placement = "outside",
      axis.title = element_text(size = 10),
      axis.text = element_text(size = 10, color = "black"),
      legend.position = c(1, 0.2),
      legend.justification = c(1, 0),
      legend.key.height = unit(6, "pt")
    )
)

class.levels <- unique(mpg2$class)
class.colors <- RColorBrewer::brewer.pal(
  n = length(class.levels),
  name = "Dark2"
) |> 
  setNames(nm = class.levels)
mpg2$class.colored <- glue.color.string(class.colors[mpg2$class], mpg2$class)
mpg2$model.colored <- glue.color.string(class.colors[mpg2$class], mpg2$model)

p3 <- mpg2 |>
  mutate(
    class2 = ifelse(
      class == "minivan-MPV",
      class.colored,
      glue.color.string("#ffffff00", class, cex = 0)
    ),
    class3 = ifelse(
      class == "subcompact-B-segment",
      class.colored,
      glue.color.string("#ffffff00", class, cex = 0)
    ),
    class4 = ifelse(
      class %in% c("minivan-MPV", "subcompact-B-segment"),
      glue.color.string("#ffffff00", class, cex = 0),
      class.colored
    )
  ) |>
  # View()
  ggplot(
    aes(x = hwy, y = model.colored)
  ) +
  geom_point(size = 1) +
  scale_x_continuous(
    expand = expansion(mult = c(0, 0.06))
  ) +
  facet_grid(
    class2 + class4 + class3 ~ .,
    scales = "free_y",
    space = "free_y",
    switch = "y"
  ) +
  labs(
    x = "Highway miles per gallon",
    y = NULL
  ) +
  coord_cartesian(
    xlim = c(10, 50),
    clip = "off"
  ) +
  theme(
    # strip.background = element_rect(color = "black", fill = NA),
    strip.text = element_markdown(
      size = 10,
      fill = alpha("lightgrey", 0.2), # alpha 设置透明度
      angle = 90,
      padding = unit(2, "pt"),
      r = unit(2, "pt")
    ),
    axis.text.y = element_markdown(size = 10)
  )
p3

# ggview::ggview(p3, width = 5, height = 7, dpi = 300)

ggsave(
  "two-rows-facet-richtext.png",
  p3,
  width = 5,
  height = 7,
  dpi = 600
)