ggplot中设置shape美学映射但形状图例未显示的技术问题

使用ggplot创建包含color和shape两种美学映射的图表时，颜色图例正常显示，但形状图例未出现。

原绘图代码 #

sav %>% 
  ggplot(aes(x = Subject, y = Percentage, color = Subject, shape = Location)) +
  geom_point(data = filter(sav, Location == "IRELAND"), shape = "square",  size = 3) + 
  geom_point(data = filter(sav, Location == "OECD"), shape = "diamond",  size = 3) +
  geom_point(data = filter(sav, !Location %in% c("OECD", "IRELAND")), shape = "circle", 
             size = 2, position = "jitter") +
  coord_flip() +
  theme(legend.position = "top")+
  theme_light()

数据dput输出 #

structure(list(Location = c("CANADA", "CANADA", "FRANCE", "FRANCE", 
"GERMANY", "GERMANY", "IRELAND", "IRELAND", "ITALY", "ITALY", 
"NETHERLANDS", "NETHERLANDS", "SPAIN", "SPAIN", "UNITED STATES", 
"UNITED STATES", "OECD", "OECD", "CANADA", "ITALY", "SPAIN", 
"FRANCE", "IRELAND", "UNITED STATES", "NETHERLANDS", "OECD", 
"GERMANY"), Subject = c("Food", "Total", "Food", "Total", "Food", 
"Total", "Food", "Total", "Food", "Total", "Food", "Total", "Food", 
"Total", "Food", "Total", "Food", "Total", "Total_Minus_Food_Energy", 
"Total_Minus_Food_Energy", "Total_Minus_Food_Energy", "Total_Minus_Food_Energy", 
"Total_Minus_Food_Energy", "Total_Minus_Food_Energy", "Total_Minus_Food_Energy", 
"Total_Minus_Food_Energy", "Total_Minus_Food_Energy"), Frequency = c("Monthly", 
"Monthly", "Monthly", "Monthly", "Monthly", "Monthly", "Monthly", 
"Monthly", "Monthly", "Monthly", "Monthly", "Monthly", "Monthly", 
"Monthly", "Monthly", "Monthly", "Monthly", "Monthly", "Monthly", 
"Monthly", "Monthly", "Monthly", "Monthly", "Monthly", "Monthly", 
"Monthly", "Monthly"), Time = c("2022-12", "2022-12", "2022-12", 
"2022-12", "2022-12", "2022-12", "2022-12", "2022-12", "2022-12", 
"2022-12", "2022-12", "2022-12", "2022-12", "2022-12", "2022-12", 
"2022-12", "2022-12", "2022-12", "2022-12", "2022-12", "2022-12", 
"2022-12", "2022-12", "2022-12", "2022-12", "2022-12", "2022-12"
), Percentage = c(11.02015, 6.319445, 12.86678, 5.850718, 19.75631, 
8.550855, 11.74636, 8.224299, 13.14815, 11.63227, 16.7983, 9.586879, 
15.68565, 5.70769, 11.88275, 6.454401, 15.60381, 9.438622, 5.58275, 
4.469475, 4.442303, 3.36004, 4.999758, 5.707835, 6.034873, 7.221961, 
5.05511)), class = "data.frame", row.names = c(NA, -27L))

图表截图 #

你在每个geom_point()中手动指定了shape参数（比如shape = "square"），这属于直接设置图形属性，而非基于数据的美学映射。这种操作会覆盖全局aes()中shape = Location的映射规则，导致ggplot无法识别需要为Location生成对应的形状图例。

推荐采用符合ggplot语法的方式：保留全局的shape = Location映射，通过scale_shape_manual()来指定每个Location对应的形状，同时可以通过size的映射或手动调整来区分不同组的点大小。

修改后的代码 #

library(dplyr)
library(ggplot2)

sav %>% 
  ggplot(aes(x = Subject, y = Percentage, color = Subject, shape = Location, size = Location)) +
  geom_point(position = position_jitter(height = 0)) +
  # 指定每个Location对应的形状
  scale_shape_manual(values = c(
    CANADA = 16, FRANCE = 16, GERMANY = 16, ITALY = 16, NETHERLANDS = 16, 
    SPAIN = 16, `UNITED STATES` = 16, IRELAND = 15, OECD = 18
  )) +
  # 指定每个Location对应的点大小
  scale_size_manual(values = c(
    CANADA = 2, FRANCE = 2, GERMANY = 2, ITALY = 2, NETHERLANDS = 2, 
    SPAIN = 2, `UNITED STATES` = 2, IRELAND = 3, OECD = 3
  )) +
  coord_flip() +
  theme(legend.position = "top") +
  theme_light()

简化方案（如果不需要单独设置每个国家的形状） #

如果只需要区分IRELAND/OECD和其他国家，也可以先对Location进行分组，再映射：

sav %>% 
  mutate(Location_group = case_when(
    Location == "IRELAND" ~ "IRELAND",
    Location == "OECD" ~ "OECD",
    TRUE ~ "Other Countries"
  )) %>%
  ggplot(aes(x = Subject, y = Percentage, color = Subject, shape = Location_group, size = Location_group)) +
  geom_point(position = position_jitter(height = 0)) +
  scale_shape_manual(values = c("Other Countries" = 16, IRELAND = 15, OECD = 18)) +
  scale_size_manual(values = c("Other Countries" = 2, IRELAND = 3, OECD = 3)) +
  coord_flip() +
  theme(legend.position = "top") +
  theme_light()

这两种方法都能让ggplot自动生成形状图例，同时保持你需要的点样式区分。

内容的提问来源于stack exchange，提问作者Smitesh Patil