使用ggplot2时已在aes()中设置color，但图例仍不显示

问题背景 #

使用ggplot2绘制折线图时图例无法显示，已尝试在aes()内定义color参数，但效果不符合预期。

数据集 #

date     `N Rx AFTN` `N Tx AFTN`  SumAFTN `N Rx AMHS` `N Tx AMHS`  SumAMHS   SUM_Tx SUM_Total   SUM_Rx
  <chr>          <int>       <int>    <dbl>       <int>       <int>    <dbl>    <dbl>     <dbl>    <dbl>
1 2018_M12    22844731    36682853 59527584     5932852     5942479 11875331 42625332  71402915 28777583
2 2019_M12    31858229    52049649 83907878    23412883    25242891 48655774 77292540 132563652 55271112
3 2020_M12    12788348    27191283 39979631    14769682    18274489 33044171 45465772  73023802 27558030
4 2021_M12    11718972    36313167 48032139    22200241    24076705 46276946 60389872  94309085 33919213
5 2022_M12    13362373    46255935 59618308    32268850    31050963 63319813 77306898 122938121 45631223
6 2023_M12    14479914    50957248 65437162    35808014    34036231 69844245 84993479 135281407 50287928

原绘图代码 #

print(ggplot(all_df))+
  geom_point(aes(x=date,y=SUM_Total/1000000))+
  geom_line(aes(x=date,y=SUM_Total/1000000,color=SUM_Total),color="blue",group=1)+
  geom_line(aes(x=date,y=SUM_Rx/1000000, color=SUM_Rx),color="green",group=1) +
  geom_line(aes(x=date,y=SUM_Tx/1000000,color=SUM_Tx),color="red",group=1)

数据集dput输出 #

structure(list(date = c("2018_M12", "2019_M12", "2020_M12", "2021_M12", "2022_M12", "2023_M12"), N Rx AFTN = c(22844731L, 31858229L, 12788348L, 11718972L, 13362373L, 14479914L), N Tx AFTN = c(36682853L, 52049649L, 27191283L, 36313167L, 46255935L, 50957248L), SumAFTN = c(59527584, 83907878, 39979631, 48032139, 59618308, 65437162), N Rx AMHS = c(5932852L, 23412883L, 14769682L, 22200241L, 32268850L, 35808014L), N Tx AMHS = c(5942479L, 25242891L, 18274489L, 24076705L, 31050963L, 34036231L), SumAMHS = c(11875331, 48655774, 33044171, 46276946, 63319813, 69844245), SUM_Rx = c(28777583, 55271112, 27558030, 33919213, 45631223, 50287928), SUM_Tx = c(42625332, 77292540, 45465772, 60389872, 77306898, 84993479), SUM_Total = c(71402915, 132563652, 73023802, 94309085, 122938121, 135281407)), row.names = c(NA, -6L), class = c("tbl_df", "tbl", "data.frame"), .internal.selfref = <pointer: 0x5613f6742840>, sorted = "date")

问题原因 #

1. 颜色映射逻辑错误：原代码中aes(color=SUM_Total)是将数值本身作为颜色映射依据，而非按系列（SUM_Total、SUM_Rx、SUM_Tx）区分颜色，不符合图例生成的逻辑。
2. 固定颜色覆盖映射：在geom_line()外部直接指定color="blue"这类固定颜色，会覆盖aes()内的颜色映射，导致ggplot无法识别系列分类，进而不生成图例。

解决方案 #

方案1：调整原代码，正确设置颜色映射 #

将系列名称作为分类变量放入aes()的color参数中，移除外部固定颜色，再通过scale_color_manual()指定对应颜色：

ggplot(all_df) +
  geom_point(aes(x = date, y = SUM_Total/1000000)) +
  geom_line(aes(x = date, y = SUM_Total/1000000, color = "SUM_Total"), group = 1) +
  geom_line(aes(x = date, y = SUM_Rx/1000000, color = "SUM_Rx"), group = 1) +
  geom_line(aes(x = date, y = SUM_Tx/1000000, color = "SUM_Tx"), group = 1) +
  scale_color_manual(values = c("SUM_Total" = "blue", "SUM_Rx" = "green", "SUM_Tx" = "red")) +
  labs(y = "数值（百万）", color = "系列")

方案2：转换为长格式数据（ggplot推荐方式） #

ggplot更适配长格式数据，先将宽格式数据转换为长格式，再绘图，代码更简洁易维护：

library(tidyr)

# 转换为长格式
long_df <- all_df %>%
  select(date, SUM_Total, SUM_Rx, SUM_Tx) %>%
  pivot_longer(cols = -date, names_to = "series", values_to = "value") %>%
  mutate(value = value / 1000000)

# 绘图
ggplot(long_df, aes(x = date, y = value, color = series, group = series)) +
  geom_line() +
  geom_point(data = long_df %>% filter(series == "SUM_Total")) + # 仅为SUM_Total添加散点
  scale_color_manual(values = c("SUM_Total" = "blue", "SUM_Rx" = "green", "SUM_Tx" = "red")) +
  labs(y = "数值（百万）", color = "系列")

内容的提问来源于stack exchange，提问作者Tassos Pan