如何旋转ggplot2中geom_abline的图例符号？

我想把ggplot2中geom_abline的图例键改成水平样式，最好能根据线条的斜率对应旋转。以下是可复现代码：

library(ggplot2)
df <- structure(list(x = c(0, 0, 0), y = c(0, 0, 0), slope = c(0.5, 1, 0.75), group = c("A", "B", "C")), class = "data.frame", row.names = c(NA, -3L))

p = ggplot(df, mapping = aes(x = x, y = y)) +
  geom_point() +
  geom_abline(df, mapping = aes(slope = slope, intercept = 0, color = factor(group))) +
  coord_cartesian(xlim = c(0, 3), ylim = c(0, 3)) 
p

我试过用grid函数修改，但会同时旋转绘图中的直线和图例键，现在想请教如何只旋转geom_abline的图例键？

方案1：将图例键改为水平样式 #

通过自定义图例的draw_key函数，替换默认的draw_key_abline，让图例里的线条保持水平：

library(ggplot2)
library(grid)

# 自定义水平线条图例键绘制函数
draw_key_hline <- function(data, params, size) {
  segmentsGrob(
    x0 = 0.1, y0 = 0.5, x1 = 0.9, y1 = 0.5,
    gp = gpar(
      col = alpha(data$colour, data$alpha),
      lwd = data$size * .pt,
      lty = data$linetype,
      lineend = "butt"
    )
  )
}

# 绘图时指定自定义的图例键绘制逻辑
p <- ggplot(df, aes(x = x, y = y)) +
  geom_point() +
  geom_abline(aes(slope = slope, intercept = 0, color = factor(group)),
              key_glyph = draw_key_hline) +
  coord_cartesian(xlim = c(0, 3), ylim = c(0, 3))
p

方案2：根据斜率匹配旋转图例键 #

如果要让图例键的线条斜率和绘图区的直线完全一致，可以通过draw_key函数获取映射的斜率值，计算对应旋转角度后绘制：

library(ggplot2)
library(grid)

# 自定义匹配斜率的图例键绘制函数
draw_key_sloped_line <- function(data, params, size) {
  # 将斜率转换为旋转角度（弧度转角度）
  angle <- atan(data$slope) * (180 / pi)
  # 创建带旋转角度的线条
  segmentsGrob(
    x0 = 0.1, y0 = 0.5, x1 = 0.9, y1 = 0.5,
    gp = gpar(
      col = alpha(data$colour, data$alpha),
      lwd = data$size * .pt,
      lty = data$linetype,
      lineend = "butt"
    ),
    vp = viewport(angle = angle)
  )
}

# 绘图时应用自定义函数，确保slope被纳入映射数据
p <- ggplot(df, aes(x = x, y = y)) +
  geom_point() +
  geom_abline(aes(slope = slope, intercept = 0, color = factor(group)),
              key_glyph = draw_key_sloped_line) +
  coord_cartesian(xlim = c(0, 3), ylim = c(0, 3))
p

这种方式只会修改图例区域的线条角度，绘图区的直线完全不受影响，实现了斜率匹配的图例键效果。

内容的提问来源于stack exchange，提问作者Quinten