使用地图包在R中为地图图中的颜色编写if-then语句

install.packages("ggplot2")
install.packages("maps")

library(ggplot2)
library(maps)

us_map <- map_data("state")

data <- data.frame(region = c("New York", "California", "Texas"),
                   value = c(10, 20, 30))

merged_data <- merge(us_map, data, by.x = "region", by.y = "region", all.x = TRUE)

ggplot() +
  geom_polygon(data = merged_data, aes(x = long, y = lat, group = group, fill = value), color = "black") +
  coord_map()

ggplot() +
  geom_polygon(data = merged_data, aes(x = long, y = lat, group = group, fill = value), color = "black") +
  coord_map() +
  scale_fill_manual(values = ifelse(merged_data$value > 20, "red", "blue"))