如何为ggplot主数据后方的ribbon单独设置图例？

当为ribbon映射fill生成独立图例后，折线类图层（观测值、预测值）的图例项会出现深灰色填充遮挡内容，以下是几种可行的解决办法：

方法1：用override.aes修正线图例的填充样式 #

通过guides()函数的override.aes参数，强制移除线图例的填充色，避免遮挡：

library(ggplot2)

# 模拟示例数据
set.seed(123)
df <- data.frame(
  x = 1:10,
  obs = rnorm(10, 5, 1),
  pred = rnorm(10, 5, 0.8),
  lower = rnorm(10, 4, 0.5),
  upper = rnorm(10, 6, 0.5)
)

ggplot(df, aes(x = x)) +
  geom_ribbon(aes(ymin = lower, ymax = upper, fill = "预测误差区间"), alpha = 0.3) +
  geom_line(aes(y = obs, color = "观测值"), size = 1) +
  geom_line(aes(y = pred, color = "预测值"), size = 1, linetype = "dashed") +
  # 强制线图例填充为透明
  guides(
    color = guide_legend(override.aes = list(fill = NA)),
    fill = guide_legend(title = NULL)
  ) +
  scale_fill_manual(values = "lightblue") +
  scale_color_manual(values = c("观测值" = "black", "预测值" = "red")) +
  theme_minimal()

方法2：仅在ribbon内部映射fill #

不在全局aes()中设置fill，只在geom_ribbon()内部单独映射，同时统一设置图例按键背景透明：

ggplot(df, aes(x = x)) +
  geom_ribbon(aes(ymin = lower, ymax = upper, fill = "预测误差区间"), alpha = 0.3) +
  geom_line(aes(y = obs, color = "观测值"), size = 1) +
  geom_line(aes(y = pred, color = "预测值"), size = 1, linetype = "dashed") +
  scale_fill_manual(name = "", values = "lightblue") +
  scale_color_manual(name = "", values = c("观测值" = "black", "预测值" = "red")) +
  theme(legend.key = element_rect(fill = NA))

方法3：用ggnewscale分离fill图例 #

借助ggnewscale包创建独立的fill图例尺度，彻底避免和color图例互相干扰：

library(ggnewscale)

ggplot(df, aes(x = x)) +
  geom_line(aes(y = obs, color = "观测值"), size = 1) +
  geom_line(aes(y = pred, color = "预测值"), size = 1, linetype = "dashed") +
  scale_color_manual(name = "", values = c("观测值" = "black", "预测值" = "red")) +
  # 新建独立的fill图例尺度
  new_scale_fill() +
  geom_ribbon(aes(ymin = lower, ymax = upper, fill = "预测误差区间"), alpha = 0.3) +
  scale_fill_manual(name = "", values = "lightblue") +
  theme_minimal()

内容的提问来源于stack exchange，提问作者Margaret