如何在ggplot折线图中仅降低指定线条的透明度？

要实现仅降低灰色线条透明度、保留彩色线条鲜艳度的效果，有两种简单可行的方法：

方案一：分两次绘制线条（推荐） #

将数据拆分为「重点县（彩色线条）」和「其他县（灰色线条）」两部分，分别调用geom_line并设置不同的透明度，直观控制每类线条的样式：

ggplot() +
  # 先绘制所有非重点县的灰色线条，设置低透明度
  geom_line(
    data = IRL_counties_Covid19[!IRL_counties_Covid19$CountyName %in% c("Galway", "Dublin", "Leitrim"), ],
    aes(x=TimeStamp, y=ConfirmedC, group=CountyName),
    color = "grey", size=1.5, alpha = 0.1
  ) +
  # 再绘制三个重点县的彩色线条，保留全透明度
  geom_line(
    data = IRL_counties_Covid19[IRL_counties_Covid19$CountyName %in% c("Galway", "Dublin", "Leitrim"), ],
    aes(x=TimeStamp, y=ConfirmedC, color=CountyName),
    size=1.5, alpha = 1
  ) +
  xlab("Date") + ylab("Cumulative Cases") +
  scale_y_continuous(labels = scales::comma, breaks = seq(0, 250000, 25000)) +
  scale_x_date(date_labels = "%b %Y", date_breaks = "1 month") +
  theme_bw() +
  theme(legend.position = "bottom") +
  scale_color_manual(values = c("Galway" = "orange", "Dublin" = "green", "Leitrim" = "purple")) +
  guides(color = guide_legend(override.aes = list(size = 3))) +
  ggtitle("Cumulative COVID-19 Cases by County in Ireland (Sep 2021 - Mar 2023)")

说明 #

• 先画灰色线条、再画彩色线条，能保证彩色线条显示在上方，避免被灰色线条遮挡
• 无需修改原始数据，逻辑清晰，新手易上手

方案二：通过映射alpha参数实现 #

给原始数据新增一列标记「是否为重点县」，将alpha映射到该列，再手动设置透明度值：

# 第一步：给数据新增分类列
IRL_counties_Covid19 <- IRL_counties_Covid19 %>%
  mutate(is_highlight = ifelse(CountyName %in% c("Galway", "Dublin", "Leitrim"), "highlight", "other"))

# 第二步：绘图并映射alpha
ggplot(IRL_counties_Covid19, aes(x=TimeStamp, y=ConfirmedC, group=CountyName, color=CountyName, alpha=is_highlight)) +
  geom_line(size=1.5) +
  xlab("Date") + ylab("Cumulative Cases") +
  scale_y_continuous(labels = scales::comma, breaks = seq(0, 250000, 25000)) +
  scale_x_date(date_labels = "%b %Y", date_breaks = "1 month") +
  theme_bw() +
  theme(legend.position = "bottom") +
  # 修正颜色映射：灰色数量应为总县数减3（3个彩色县）
  scale_color_manual(
    values = c("Galway" = "orange", "Dublin" = "green", "Leitrim" = "purple", 
               rep("grey", length(unique(IRL_counties_Covid19$CountyName))-3)), 
    breaks = c("Galway", "Dublin", "Leitrim")
  ) +
  # 设置透明度映射，隐藏alpha图例避免干扰
  scale_alpha_manual(values = c("highlight" = 1, "other" = 0.1), guide = "none") +
  guides(color = guide_legend(override.aes = list(size = 3))) +
  ggtitle("Cumulative COVID-19 Cases by County in Ireland (Sep 2021 - Mar 2023)")

说明 #

• 适合需要对数据进行更多分类处理的场景
• 注意修正scale_color_manual中灰色的数量，避免颜色不匹配

内容的提问来源于stack exchange，提问作者Joseph Linane