如何将ggplot2中的离散填充等高线图例改为连续型？

我有一个关于ggplot2图例的疑问。我提供了一段可复现的R代码，当前代码生成的是离散分段的图例，但我希望将其改为平滑的连续型图例（如附图右侧样式）。代码较长但可能仅需修改一行即可实现，请问如何调整？

用户提供的原代码：

library(sf)
library(sp)

r <- raster(t((volcano[,ncol(volcano):1] - 94) * 4.95))

# Let's mock with a shapefile
poly <- st_as_sfc(st_bbox(st_as_sf(rasterToPolygons(r))))

# Sample 4 points
set.seed(3456)

sample <- st_sample(poly, 4)
sample <- st_buffer(sample, c(0.01, 0.02, 0.03))
sample <- st_sf(x=1:4, sample)
st_write(sample, "1aa.shp", append = FALSE)
# Mocked data

# Now let's start with code -------

r <- raster(t((volcano[,ncol(volcano):1] - 94) * 4.95))

# Use sf!!
pg <- st_read("1aa.shp") # loadshapfile 
plot(r)
plot(st_geometry(pg), add= TRUE,) # it appears here like first picture (left).

centile90 <- quantile(r, 0.90)
df <- as.data.frame(as(r, "SpatialPixelsDataFrame"))
colnames(df) <- c("value", "x", "y")

library(ggplot2)

mybreaks <- seq(0, 500, 50)

ggplot(df, aes(x, y, z = value)) +
geom_contour_filled(breaks = mybreaks) +
geom_contour(breaks = centile90, colour = "pink",
           size = 0.5) +
 # And here we have it
 geom_sf(data=pg, fill="black", inherit.aes = FALSE) +
scale_fill_manual(values = hcl.colors(length(mybreaks)-1, "Zissou1", rev 
= FALSE)) +
scale_x_continuous(expand = c(0, 0)) +
scale_y_continuous(expand = c(0, 0)) +
theme_classic() +
theme()

调整方案 #

要实现平滑连续的图例，核心是替换离散填充图层和手动色标为连续型选项，只需修改2处：

1. 将geom_contour_filled()替换为geom_raster(aes(fill = value), interpolate = TRUE)——直接渲染栅格数据的连续颜色，interpolate=TRUE让颜色过渡更自然
2. 将scale_fill_manual()替换为scale_fill_continuous(type = "Zissou1")——使用连续色标生成平滑渐变图例

修改后的完整代码：

library(sf)
library(sp)
library(raster)
library(ggplot2)

# 生成模拟数据（原代码保留）
r <- raster(t((volcano[,ncol(volcano):1] - 94) * 4.95))
poly <- st_as_sfc(st_bbox(st_as_sf(rasterToPolygons(r))))
set.seed(3456)
sample <- st_sample(poly, 4)
sample <- st_buffer(sample, c(0.01, 0.02, 0.03))
sample <- st_sf(x=1:4, sample)
st_write(sample, "1aa.shp", append = FALSE)

# 加载数据并绘图
r <- raster(t((volcano[,ncol(volcano):1] - 94) * 4.95))
pg <- st_read("1aa.shp") 
centile90 <- quantile(r, 0.90)
df <- as.data.frame(as(r, "SpatialPixelsDataFrame"))
colnames(df) <- c("value", "x", "y")

ggplot(df, aes(x, y)) +
  # 替换为连续栅格填充
  geom_raster(aes(fill = value), interpolate = TRUE) +
  geom_contour(aes(z = value), breaks = centile90, colour = "pink", size = 0.5) +
  geom_sf(data=pg, fill="black", inherit.aes = FALSE) +
  # 替换为连续色标
  scale_fill_continuous(type = "Zissou1") +
  scale_x_continuous(expand = c(0, 0)) +
  scale_y_continuous(expand = c(0, 0)) +
  theme_classic()

说明 #

• geom_contour_filled是基于等高线生成离散分段填充，因此图例必然是离散的；而geom_raster直接根据每个像素的数值渲染连续颜色，配合连续色标就能得到平滑渐变的图例。
• 如果需要保留原有的分段参考，也可以在scale_fill_continuous中添加breaks = mybreaks参数，让图例显示指定的刻度点，但颜色依然是平滑过渡的。

内容的提问来源于stack exchange，提问作者user_3264