ggplot使用scale_linetype_manual时仅让geom_vline图例项为竖线的问题

解决方法 #

出现该问题的核心原因是ggplot2默认会将所有映射到colour和linetype的分类合并为同一个图例，导致geom_vline的图例样式会覆盖所有同组图例条目的展示形态。可以用以下两种方案实现需求：

方案1：原生ggplot2实现（无额外依赖，推荐） #

仅需新增guides配置段，单独指定每个图例条目对应的图标样式即可，修改后完整代码如下：

library(ggplot2)
# 补充示例数据集，实际使用时可删除该行
df <- data.frame(a = seq(-1,1,0.01), b = rnorm(201), c = rnorm(201, mean = 2))

ggplot(data = df) +
  geom_line(aes(x = a, y = b, colour = "one", linetype = "one"), size = 2) +
  geom_line(aes(x = a, y = c, colour = "two", linetype = "two"), size = 2) +
  # 开启geom_vline的图例展示
  geom_vline(aes(xintercept = 0.5, colour = "three", linetype = "three"), alpha = 1, show.legend = T) +
  scale_colour_manual("",
                      breaks = c("one", "two", "three"),
                      values = c("blue", "red", "green")) +
  scale_linetype_manual("",
                        breaks = c("one", "two", "three"),
                        values = c("dashed", "solid", "solid")) +
  # 新增的图例样式覆盖配置
  guides(
    colour = guide_legend(override.aes = list(
      # 前两个条目用折线默认的横线图标，第三个用竖线图标
      key_glyph = c("path", "path", "vline"),
      size = c(2,2,2)
    )),
    linetype = guide_legend(override.aes = list(
      key_glyph = c("path", "path", "vline"),
      size = c(2,2,2)
    ))
  ) +
  theme_bw() +
  theme(legend.key.size = unit(2, "cm"),
        legend.position = c(0.05, 0.995), legend.justification = c(0.05, 0.995))

该方案支持ggplot2 3.3.0及以上版本使用。

方案2：用ggnewscale包拆分图例 #

如果需要更灵活的图例自定义，可以用ggnewscale包将折线和竖线的比例尺拆分为独立组，避免样式互相干扰：

library(ggplot2)
library(ggnewscale)
# 补充示例数据集，实际使用时可删除该行
df <- data.frame(a = seq(-1,1,0.01), b = rnorm(201), c = rnorm(201, mean = 2))

ggplot(data = df) +
  geom_line(aes(x = a, y = b, colour = "one", linetype = "one"), size = 2) +
  geom_line(aes(x = a, y = c, colour = "two", linetype = "two"), size = 2) +
  scale_colour_manual("", breaks = c("one", "two"), values = c("blue", "red")) +
  scale_linetype_manual("", breaks = c("one", "two"), values = c("dashed", "solid")) +
  # 插入新的颜色、线型比例尺，专门给竖线使用
  new_scale_colour() +
  new_scale("linetype") +
  geom_vline(aes(xintercept = 0.5, colour = "three", linetype = "three"), alpha = 1, show.legend = T) +
  scale_colour_manual("", breaks = "three", values = "green") +
  scale_linetype_manual("", breaks = "three", values = "solid") +
  theme_bw() +
  theme(legend.key.size = unit(2, "cm"),
        legend.position = c(0.05, 0.995), legend.justification = c(0.05, 0.995))

使用该方案需要先安装ggnewscale包：install.packages("ggnewscale")

内容的提问来源于stack exchange，提问作者salomon