如何在ggplot2中为多个图层添加自定义图例？

library(ggplot2)

# 创建数据集
data <- data.frame(
  x = c(1, 2, 3, 4, 5),
  y1 = c(1, 4, 9, 16, 25),
  y2 = c(1, 2, 3, 4, 5)
)

# 创建第一个图层
layer1 <- ggplot(data, aes(x, y1)) +
  geom_line(color = "blue") +
  geom_point(color = "blue") +
  labs(title = "Layer 1")

# 创建第二个图层
layer2 <- ggplot(data, aes(x, y2)) +
  geom_line(color = "red") +
  geom_point(color = "red") +
  labs(title = "Layer 2")

# 将两个图层合并
combined_plot <- layer1 + layer2

# 添加自定义图例
combined_plot + guides(color = guide_legend(title = "图例标题"))