如何在ggplot中添加图例？

library(ggplot2)

# 创建一个数据框
data <- data.frame(
  x = c(1, 2, 3),
  y = c(4, 5, 6),
  group = c("A", "B", "C")
)

# 创建一个ggplot对象，并指定x、y变量和分组变量
p <- ggplot(data, aes(x, y, color = group))

# 添加散点图层
p <- p + geom_point()

# 设置颜色映射关系和图例标题
p <- p + scale_color_manual(values = c("red", "blue", "green"),
                             labels = c("Group A", "Group B", "Group C"),
                             name = "Group")

# 显示图形
print(p)