ggplot：如何在geom_polygon中生成渐变填充

Question

这应该相当容易，但我找不到自己的方式。

tri_fill <- structure(
  list(x= c(0.75, 0.75, 2.25, 3.25), 
       y = c(40, 43, 43, 40)), 
  .Names = c("x", "y"),
  row.names = c(NA, -4L), class = "data.frame",Integrated=NA, Related=NA)

# install.packages("ggplot2", dependencies = TRUE)
require(ggplot2)


  ggplot(data=tri_fill,aes(x=x, y=y))+ 
      geom_polygon() + 
      scale_fill_gradient(limits=c(1, 4), low = "lightgrey", high = "red")

我想要的是沿着x轴的渐变，但是上面我只得到一个带有渐变的图例和带有实心填充的多边形。

Answer 1

当你有一个相对简单的多边形时，这是一个可能的解决方案。我们创建了许多线段，并通过渐变为它们着色，而不是多边形。因此，结果看起来像一个带有渐变的多边形。

#create data for 'n'segments
n_segs <- 1000

#x and xend are sequences spanning the entire range of 'x' present in the data
newpolydata <- data.frame(xstart=seq(min(tri_fill$x),max(tri_fill$x),length.out=n_segs))
newpolydata$xend <- newpolydata$xstart


#y's are a little more complicated: when x is below changepoint, y equals max(y)
#but when x is above the changepoint, the border of the polygon
#follow a line according to the formula y= intercept + x*slope.

#identify changepoint (very data/shape dependent)
change_point <-  max(tri_fill$x[which(tri_fill$y==max(tri_fill$y))])

#calculate slope and intercept
slope <- (max(tri_fill$y)-min(tri_fill$y))/ (change_point - max(tri_fill$x))
intercept <- max(tri_fill$y)

#all lines start at same y
newpolydata$ystart <- min(tri_fill$y)

#calculate y-end
newpolydata$yend <- with(newpolydata, ifelse (xstart <= change_point,
                      max(tri_fill$y),intercept+ (xstart-change_point)*slope))

p2 <- ggplot(newpolydata) +
  geom_segment(aes(x=xstart,xend=xend,y=ystart,yend=yend,color=xstart)) +
  scale_color_gradient(limits=c(0.75, 4), low = "lightgrey", high = "red")
p2 #note that I've changed the lower border of the gradient.

编辑：上面的解决方案是有效的，如果只需要一个带渐变的多边形，但是，正如评论中指出的那样，当你计划将一个东西映射到填充而另一个东西要着色时，这会产生问题，因为每个＆ ＃39; AES＆＃39;只能使用一次。因此，我已经修改了解决方案，不是绘制线条，而是绘制（非常薄的）可以有填充的多边形。

#for each 'id'/polygon, four x-variables and four y-variable
#for each polygon, we start at lower left corner, and go to upper left, upper right and then to lower right.


n_polys <- 1000
#identify changepoint (very data/shape dependent)
change_point <-  max(tri_fill$x[which(tri_fill$y==max(tri_fill$y))])

#calculate slope and intercept
slope <- (max(tri_fill$y)-min(tri_fill$y))/ (change_point - max(tri_fill$x))
intercept <- max(tri_fill$y)
#calculate sequence of borders: x, and accompanying lower and upper y coordinates
x_seq <- seq(min(tri_fill$x),max(tri_fill$x),length.out=n_polys+1)
y_max_seq <- ifelse(x_seq<=change_point, max(tri_fill$y), intercept + (x_seq - change_point)*slope)
y_min_seq <- rep(min(tri_fill$y), n_polys+1)

#create polygons/rectangles
poly_list <- lapply(1:n_polys, function(p){
  res <- data.frame(x=rep(c(x_seq[p],x_seq[p+1]),each=2),
                    y = c(y_min_seq[p], y_max_seq[p:(p+1)], y_min_seq[p+1]))
  res$fill_id <- x_seq[p]
  res
}
)

poly_data <- do.call(rbind, poly_list)

#plot, allowing for both fill and color-aes
p3 <- ggplot(tri_fill, aes(x=x,y=y))+
  geom_polygon(data=poly_data, aes(x=x,y=y, group=fill_id,fill=fill_id)) +
  scale_fill_gradient(limits=c(0.75, 4), low = "lightgrey", high = "red") +
  geom_point(aes(color=factor(y)),size=5)
p3

Answer 2

找到的答案 in this question 关于三角形的热图实现了相同的效果，可用于任何复杂的多边形形状。