ggplot自定义图例符号：旋转key_glyph实现贸易流图图例样式

问题背景 #

我想要复刻Chris Canipe的贸易流图样式，已完成基础地图绘制，但图例样式无法匹配需求：

• 使用draw_key_vline效果接近，但需要将线条旋转90度
• 使用draw_key_rect时，图例符号大小无法随trade_val（贸易额）缩放

解决方案 #

核心思路是自定义图例符号绘制函数，修改默认draw_key_vline的线条方向，同时保留linewidth与trade_val的缩放关联。

完整修改代码 #

library(tidyverse)
library(ggarrow)
library(rnaturalearth)

# 自定义横向线条图例函数：替换竖线为横线
draw_key_hline <- function(data, params, size) {
  grid::segmentsGrob(
    x0 = 0.1, x1 = 0.9, y0 = 0.5, y1 = 0.5,
    gp = grid::gpar(
      col = alpha(data$colour %||% params$colour %||% "black", data$alpha %||% params$alpha),
      lwd = (data$linewidth %||% params$linewidth %||% 1) * .pt,
      lty = data$linetype %||% params$linetype %||% 1,
      lineend = "butt"
    )
  )
}

countries <- ne_countries(scale = "medium", returnclass = "sf")
  
data <- data.frame(
    From = c("Australia", "Belize", "China", "Cook Islands", "Fiji"),
    from_x   = c(133.77514, -88.49765, 104.19540, -159.77767, 179.41441),
    from_y   = c(-25.274398, 17.189877, 35.861660, -21.236736, -16.578193),
    to_y   = c(40.67778, 40.67778, 40.67778, 40.67778, 40.67778),
    to_x = c(-74.04425, -74.04425, -74.04425, -74.04425, -74.04425),
    trade_val = c(275, 66, 126755, 700, 19000)
)


ggplot(data = data)+
  geom_sf(data = countries)+
  geom_arrow_curve(
    data = data, 
    show.legend = T, 
    aes(x = from_x, y = from_y, xend = to_x, yend = to_y, linewidth = trade_val, alpha = trade_val),
    angle = 90, lineend = "butt", length = unit(0.5, "cm"), 
    colour = "#08519B", 
    key_glyph = draw_key_hline  # 指定自定义图例函数
  )+
  theme(
    legend.position = "top",
    legend.text.position = "bottom",
    legend.key.spacing.x = unit(-1.5, "cm"),
    legend.text = element_text(margin = margin(r = 0, unit = "pt")),
    panel.background = element_rect(fill = 'white', colour = 'white'),
    plot.background = element_rect(fill = 'white', colour = 'white'),
    legend.title = element_text(hjust = 0.5)
  )+
  guides(
    alpha = guide_legend(title = "Trade flow", label.position = "bottom"),
    linewidth = guide_legend(
      title = "Trade flow", 
      title.position = "top",
      label.position = "bottom", 
      keywidth = 6
    ),
    # 合并alpha与linewidth图例，避免重复
    override.aes = list(alpha = 1)
  )

关键说明 #

1. 自定义draw_key_hline函数：

   • 基于draw_key_vline改造，将竖线逻辑改为横向线段（x从0.1到0.9，y固定为0.5），实现目标图例的横向线条样式
   • 保留了linewidth的参数映射，确保图例线条宽度随trade_val同步缩放

2. 图例合并优化：

   • 通过override.aes = list(alpha = 1)固定透明度，让alpha和linewidth共享同一组图例，避免重复显示
   • 保留原有主题配置，维持图例的布局和视觉风格

内容的提问来源于stack exchange，提问作者Rupert Stacy