如何在类似热图的样式中在ggplot2中的地图上绘制样方数

# load libraries
library(ggplot2)
library(ggmap)
library(gridExtra)

# create sample data
df <- data.frame(lon=c(-80.18, -95.96, -74.05, -71.05, -74.00),
                 lat=c(25.77, 36.56, 42.36, 47.60, 29.44),
                 count=c(30, 10, 20, 15, 25))

# create map
map <- get_map(location = c(lon = mean(df$lon), lat = mean(df$lat)),
               zoom = 10,
               maptype = "roadmap",
               source = "google",
               color = colorRampPalette(c("blue", "white", "red"))(n = 3))

# create heatmap
ggmap(map) +
  geom_tile(data = df, aes(x = lon, y = lat, fill = count), color = "white") +
  scale_fill_gradient2(low = "blue", high = "red", space = "Lab", guide = "colorbar") +
  labs(title = paste0("Heatmap of sample data")) +
  coord_map() +
  theme_bw() +
  theme(panel.border = element_blank(), panel.grid = element_blank(),
        axis.text.x = element_text(size = 12), axis.text.y = element_text(size = 12),
        axis.title.x = element_blank(), axis.title.y = element_blank())