ggplot2中scale_fill_gradient2未显示low颜色，如何呈现三色渐变？

我遇到的问题类似相关场景，但使用scale_fill_gradient2()时能看到mid颜色（橙色），却完全看不到low颜色（绿色）。

• 绿色缺失的图表：
  

• 所用代码：

library(tidyverse)
library(forcats)

df %>% 
  ggplot(aes(y = prop, x = fct_rev(var),  
             fill = as.integer(df$var))) +  ## 把离散变量转成连续变量的临时办法
  geom_bar(stat = "identity", width = 0.3) +
  scale_fill_gradient2(name = "category", 
                        low = "green", 
                       mid= "orange",
                        high = "purple",
                       space = "Lab",
                #       midpoint = 0,
                       guide= 'legend',
              #  limits = c(-7, 7),   ## 试过其他帖子的解决方案，但没用
              ) +
  coord_flip(clip = "off")

核心问题：为什么绿色完全看不到？

• 数据结构：

structure(list(var = structure(1:7, .Label = c("VAR_A", "VAR_B", 
"VAR_C", "VAR_D", "VAR_E", "VAR_F", "VAR_G"), class = "factor"), 
    x = structure(c(2L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("no", 
"yes"), class = "factor"), n = c(7L, 30L, 49L, 48L, 47L, 
39L, 21L), prop = c(13.73, 58.82, 96.08, 94.12, 92.16, 76.47, 
41.18)), class = c("grouped_df", "tbl_df", "tbl", "data.frame"
), row.names = c(NA, -7L), groups = structure(list(var = structure(1:7, .Label = c("VAR_A", 
"VAR_B", "VAR_C", "VAR_D", "VAR_E", "VAR_F", "VAR_G"), class = "factor"), 
    .rows = structure(list(1L, 2L, 3L, 4L, 5L, 6L, 7L), ptype = integer(0), class = c("vctrs_list_of", 
"vctrs_vctr", "list"))), class = c("tbl_df", "tbl", "data.frame"
), row.names = c(NA, -7L), .drop = TRUE))

问题原因及解决办法 #

原因分析

1. 你用来映射填充色的as.integer(df$var)取值范围是1到7，scale_fill_gradient2()默认midpoint为该范围的中点（即4），绿色对应最小值1，橙色对应4，紫色对应7。
2. 你设置了guide='legend'，ggplot对连续颜色生成离散图例时，默认断点可能未包含最小值1，导致绿色未在图例和可视化中显现。
3. 之前尝试limits=c(-7,7)但未调整midpoint，默认midpoint=0时，所有数据（1-7）都落在midpoint到high区间，绿色自然不会显示。

解决办法

方案1：改用颜色条图例（推荐）

把guide='legend'改为默认的guide='colorbar'，完整显示连续颜色过渡：

library(tidyverse)
library(forcats)

df %>% 
  ggplot(aes(y = prop, x = fct_rev(var),  
             fill = as.integer(var))) +  # 无需df$，直接引用变量即可
  geom_bar(stat = "identity", width = 0.3) +
  scale_fill_gradient2(name = "category", 
                        low = "green", 
                        mid = "orange",
                        high = "purple",
                        space = "Lab") + # 默认guide为colorbar，可省略
  coord_flip(clip = "off")

方案2：保留图例并手动指定断点

如果需要用离散图例，手动指定包含最小值1的断点：

df %>% 
  ggplot(aes(y = prop, x = fct_rev(var),  
             fill = as.integer(var))) +
  geom_bar(stat = "identity", width = 0.3) +
  scale_fill_gradient2(name = "category", 
                        low = "green", 
                        mid = "orange",
                        high = "purple",
                        space = "Lab",
                        guide = guide_legend(breaks = c(1,4,7))) + # 指定包含绿色对应的1
  coord_flip(clip = "off")

内容的提问来源于stack exchange，提问作者Larissa Cury