能否将igraph的Arrows箭头样式应用于ggplot的线段与轴线？

可以实现，核心思路是利用igraph的内部箭头绘制函数igraph:::igraph.Arrows，结合ggplot2基于grid的绘图特性，以下是两种常用方法：

方法一：在ggplot绘图后直接叠加箭头 #

先绘制好ggplot图形，再调用igraph的箭头函数在当前绘图设备上叠加箭头，适用于快速需求：

library(ggplot2)
library(igraph)

# 绘制基础ggplot图
p <- qplot(0, 0, xlim = c(0, 1), ylim = c(0, 1), xlab = "", ylab = "") + theme_classic()
print(p)

# 调用igraph箭头函数绘制自定义样式箭头
iArrows <- igraph:::igraph.Arrows
iArrows(c(0, 0, 0.5), c(0, 0, 0.5), c(0, 1, 1), c(1, 0, 1), h.lwd = 2, sh.lwd = 2, width = 1, size = 1.5)

方法二：自定义ggplot图层（更贴合ggplot语法） #

封装一个自定义的geom_igraph_arrow图层，这样可以像使用原生geom_segment一样添加igraph风格箭头：

library(ggplot2)
library(igraph)
library(grid)

# 定义自定义geom的核心逻辑
GeomIgraphArrow <- ggproto("GeomIgraphArrow", Geom,
  required_aes = c("x", "y", "xend", "yend"),
  default_aes = aes(h.lwd = 2, sh.lwd = 2, width = 1, size = 1.5),
  draw_panel = function(data, panel_params, coord) {
    # 转换数据坐标到ggplot的绘图空间
    coords <- coord$transform(data, panel_params)
    # 调用igraph箭头函数绘制
    iArrows <- igraph:::igraph.Arrows
    iArrows(coords$x, coords$y, coords$xend, coords$yend,
            h.lwd = coords$h.lwd, sh.lwd = coords$sh.lwd,
            width = coords$width, size = coords$size)
    # 返回空grob，避免重复绘制
    zeroGrob()
  }
)

# 封装成可调用的geom函数
geom_igraph_arrow <- function(mapping = NULL, data = NULL, stat = "identity",
                              position = "identity", na.rm = FALSE, show.legend = NA,
                              inherit.aes = TRUE, ...) {
  layer(
    geom = GeomIgraphArrow, mapping = mapping, data = data, stat = stat,
    position = position, show.legend = show.legend, inherit.aes = inherit.aes,
    params = list(na.rm = na.rm, ...)
  )
}

# 使用示例
qplot(0, 0, xlim = c(0, 1), ylim = c(0, 1), xlab = "", ylab = "") + theme_classic() +
  geom_igraph_arrow(aes(x = 0.5, y = 0.5, xend = 1, yend = 1)) +
  geom_igraph_arrow(aes(x = 0, y = 0, xend = 0, yend = 1)) +
  geom_igraph_arrow(aes(x = 0, y = 0, xend = 1, yend = 0))

替换ggplot2的轴线箭头 #

如果要把坐标轴的箭头换成igraph样式，只需隐藏原生轴线，再用自定义geom绘制轴线和箭头：

qplot(0, 0, xlim = c(0, 1), ylim = c(0, 1), xlab = "", ylab = "") + 
  theme_classic() +
  theme(axis.line = element_blank()) + # 隐藏原生轴线
  # 绘制x轴及箭头
  geom_igraph_arrow(aes(x = 0, y = 0, xend = 1, yend = 0), h.lwd = 1, sh.lwd = 1, width = 0.8, size = 1) +
  # 绘制y轴及箭头
  geom_igraph_arrow(aes(x = 0, y = 0, xend = 0, yend = 1), h.lwd = 1, sh.lwd = 1, width = 0.8, size = 1)

注意：igraph:::igraph.Arrows是igraph的内部函数，未来版本可能有变动，使用时需留意兼容性。

内容的提问来源于stack exchange，提问作者mert