解决ggplot按年份分组设颜色时多条折线合并为单条的问题

我想要制作一幅数据按十年区间（如2000-2009、2010-2019、2020+）分组，且同组折线共用统一颜色的图表。单独为各年份设置颜色时一切正常，但尝试按年份组设置颜色时，代表各具体年份的折线会被合并成单条。我希望无需逐个指定年份，让2020年及以后的所有年份折线都像第一张图那样独立显示且全部为红色，如何避免折线合并的情况？

正常运行的代码（各年份单独设置颜色） #

suppressWarnings(library(tidyverse))

set.seed(333)

start_date <- as.Date('1990-01-01')
end_date <- as.Date('2023-12-31')
total_days <- as.numeric((end_date-start_date)+1)

# 创建数据框
dat <- data.frame(
  Date = seq(start_date, end_date,'1 day'),
  Value = rnorm(total_days, 20, 4)
)

# 为数据框添加年份和年积日（DoY）
dat <- dat %>%
  mutate(Year = year(Date),
         DoY = yday(Date))

# 各年份单独设置颜色的正常代码
dat %>%
  ggplot(aes(x = DoY, y = Value, col = factor(Year))) +
  geom_line(data = subset(dat, Year == 2020), aes(colour = "2020")) +
  geom_line(data = subset(dat, Year == 2021), aes(colour = "2021")) +
  geom_line(data = subset(dat, Year == 2022), aes(colour = "2022")) +
  geom_line(data = subset(dat, Year == 2023), aes(colour = "2023")) +
  labs(x = '年积日',
       y = '数值',
       colour = '年份区间') +
  scale_x_continuous(breaks = seq(1, 10, 1), limits = c(1,10)) +
  scale_color_manual(values = c("2020" = "#225ea8",
                                "2021" = "#41b6c4",
                                "2022" = "#a1dab4",
                                "2023" = "red")) +
  theme_bw()

存在问题的代码（年份组设置颜色导致折线合并） #

# 问题代码：为何>=2020的年份折线被合并为单条？
# 期望>=2020的年份折线像上图一样独立显示且为红色
dat %>%
  ggplot(aes(x = DoY, y = Value, col = factor(Year))) +
  # geom_line() +
  # geom_line(data = subset(dat, Year >= 1990 & Year <= 1999), aes(colour = "1990-1999")) +
  # geom_line(data = subset(dat, Year >= 2000 & Year <= 2009), aes(colour = "2000-2009")) +
  # geom_line(data = subset(dat, Year >= 2010 & Year <= 2019), aes(colour = "2010-2019")) +
  geom_line(data = subset(dat, Year >= 2020), aes(colour = "2020+")) +
  labs(x = '年积日',
       y = '数值',
       colour = '年份区间') +
  scale_x_continuous(breaks = seq(1, 10, 1), limits = c(1,10)) +
  scale_color_manual(values = c("1990-1999" = "#225ea8",
                                "2000-2009" = "#41b6c4",
                                "2010-2019" = "#a1dab4",
                                "2020+" = "red")) +
  theme_bw()

^{创建于2024-02-21，使用reprex v2.0.2}

解决方案 #

问题根源是：当你在geom_line里指定aes(colour = "2020+")时，ggplot会把所有符合条件的数据归为同一个分组，最终合并成单条折线。要保留各年份独立折线且统一颜色，有两种简洁方法：

方法1：预处理数据添加年份分组列 #

先给数据新增一个标记年份区间的列，用该列映射颜色，同时保留Year作为分组依据：

dat <- dat %>%
  mutate(Year_Group = case_when(
    Year >= 1990 & Year <= 1999 ~ "1990-1999",
    Year >= 2000 & Year <= 2009 ~ "2000-2009",
    Year >= 2010 & Year <= 2019 ~ "2010-2019",
    Year >= 2020 ~ "2020+"
  ))

# 绘图代码
dat %>%
  ggplot(aes(x = DoY, y = Value, group = Year, colour = Year_Group)) +
  geom_line() +
  labs(x = '年积日',
       y = '数值',
       colour = '年份区间') +
  scale_x_continuous(breaks = seq(1, 10, 1), limits = c(1,10)) +
  scale_color_manual(values = c("1990-1999" = "#225ea8",
                                "2000-2009" = "#41b6c4",
                                "2010-2019" = "#a1dab4",
                                "2020+" = "red")) +
  theme_bw()

方法2：在geom_line中显式指定group映射 #

如果不想修改原始数据，直接在geom_line里指定group = Year，确保每个年份的折线独立：

dat %>%
  ggplot(aes(x = DoY, y = Value)) +
  geom_line(data = subset(dat, Year >= 2020), aes(group = Year, colour = "2020+")) +
  geom_line(data = subset(dat, Year >= 2010 & Year <= 2019), aes(group = Year, colour = "2010-2019")) +
  geom_line(data = subset(dat, Year >= 2000 & Year <= 2009), aes(group = Year, colour = "2000-2009")) +
  geom_line(data = subset(dat, Year >= 1990 & Year <= 1999), aes(group = Year, colour = "1990-1999")) +
  labs(x = '年积日',
       y = '数值',
       colour = '年份区间') +
  scale_x_continuous(breaks = seq(1, 10, 1), limits = c(1,10)) +
  scale_color_manual(values = c("1990-1999" = "#225ea8",
                                "2000-2009" = "#41b6c4",
                                "2010-2019" = "#a1dab4",
                                "2020+" = "red")) +
  theme_bw()

两种方法都能实现同组年份折线独立显示、颜色统一的效果，无需逐个指定年份。

内容的提问来源于stack exchange，提问作者tassones