在 position_dodge() 之后在 ggplot2 中进行注释？

Question

我正在尝试在 R 中生成一个双因子实验的图形。这包含三个级别，每个级别都有两个子级别。我想对这些组中的每一个进行注释，但找不到合理的方法。

这是一个为顶层生成注释的示例：

ggplot(mtcars, aes(x = as.factor(cyl), y = mpg)) +
  geom_point(aes(colour = as.factor(am)), position = position_dodge(0.5)) +
  annotate("text", x = 1:3, y = c(37, 25, 22),
           label = c("a", "b", "c"))

我真正想做的是注释“am”的每个级别。

这是我的天真尝试：

ggplot(mtcars, aes(x = as.factor(cyl), y = mpg)) +
  geom_point(aes(colour = as.factor(am)), position = position_dodge(0.5)) +
  annotate("text", x = 1:6, y = c(27, 37, 25, 25, 22, 17),
           label = c("a", "b", "c", "d", "e", "f"), position = position_dodge(0.5))

> Error: Unequal parameter lengths: x (6), y (6), label (6), position (3)

我尝试改变 x、y 和标签的数量。我怀疑答案在于为注释分配“am”级别，但我不知道该怎么做。谁有可行的解决方案？

Answer 1

也许改用geom_text？

library(ggplot2)

ggplot(mtcars, aes(x = cyl, y = mpg)) +
  geom_point(aes(colour = as.factor(am)), position = position_dodge(0.5)) +
  geom_text(data = data.frame(x = c(3.8, 4.2, 5.8, 6.2, 7.8, 8.2), 
                              y = c(27, 37, 25, 25, 22, 17)), 
  aes(x, y, label = c("a", "b", "c", "d", "e", "f")))

Answer 2

预先计算 y 值，而不是硬编码标签位置，并使用 aes 和 group 映射 x 位置。

# calculate desired y positions for cyl * am combinations, e.g. max 
d <- aggregate(mpg ~ am cyl, data = mtcars, max)

# some toy labels
d$lab <- letters[1:nrow(d)]

# calculate desired y positions for cyl groups, e.g. mean of am max
d2 <- aggregate(mpg ~ cyl, data = d, mean)

# some toy labels
d2$lab <-LETTERS[1:nrow(d2)]

pd <- position_dodge(width = 0.5)

ggplot(mtcars, aes(x = factor(cyl), y = mpg)) +
  geom_point(aes(colour = factor(am)), position = pd) + 
  geom_text(data = d, aes(y = mpg + 1, label = lab, group = factor(am)), position = pd) +
  geom_text(data = d2, aes(label = lab))