如何在R图中定义每个轨迹的单独调色板？

# 安装并加载必要的包
if (!require("ggplot2")) install.packages("ggplot2")
library(ggplot2)

# 示例数据
set.seed(123)
times <- seq(as.POSIXct("2020-01-01"), as.POSIXct("2020-12-31"), by="day")
df <- data.frame(time = times,
                 traj1 = rnorm(length(times)),
                 traj2 = rnorm(length(times), mean=2),
                 traj3 = rnorm(length(times), mean=-2))

# 将数据转换为长格式，以便ggplot2使用
library(tidyr)
df_long <- pivot_longer(df, cols = starts_with("traj"), names_to = "trajectory", values_to = "value")

# 绘制图形，为每个轨迹指定不同的颜色
ggplot(df_long, aes(x = time, y = value, color = trajectory)) +
  geom_line() +
  scale_color_manual(values = c("traj1" = "blue", "traj2" = "red", "traj3" = "green")) +
  theme_minimal()