用渐变填充 geom_tile

Question

我想创建一个渐变图。我想用渐变填充我的geom_tile。然而，R 不断告诉我Error: Discrete value supplied to continuous scale。

df <- data.frame(value=c(55, 40, 5),
             zz=c("A", "B", "C"))

df$lower <- df$value-2.9
df$upper <- df$value+2.9
ggplot(df, aes(x=zz, y=value, fill=zz))+
   geom_tile(aes(x=zz, y=value, fill=zz), width=0.2,height=2.9)

现在，我想用渐变（中心颜色最浓（column=value）并逐渐淡出到末端（上和下））为图块着色。

我怎样才能做到这一点？ geom_tile 是正确的 geom 吗？ 谢谢

编辑

渐变应该在图块内，请参阅 Alex Krusz 的这个示例。链接：here

Answer 1

我认为 ggplot 不适合这种用途，但这是模拟透明度渐变的一种方法。

创建插值透明度值的数据集：

library(data.table)

df <- setDT(df)
n <- 100
df.lower <- df[, .(ymin = seq(lower, value, length.out = n + 1)[1:n],
                   ymax = seq(lower, value, length.out = n + 1)[2:(n+1)],
                   alpha = seq(0, 1, length.out = n)), by = .(zz)]
df.upper <- df[, .(ymin = seq(value, upper, length.out = n + 1)[1:n],
                   ymax = seq(value, upper, length.out = n + 1)[2:(n+1)],
                   alpha = seq(1, 0, length.out = n)), by = .(zz)]
df.new <- rbind(df.lower, df.upper)
df.new$x <- as.integer(df.new$zz)
df.new$xmin <- df.new$x - 0.2
df.new$xmax <- df.new$x + 0.2

> df.new
     zz   ymin   ymax      alpha x xmin xmax
  1:  A 52.100 52.129 0.00000000 1  0.8  1.2
  2:  A 52.129 52.158 0.01010101 1  0.8  1.2
  3:  A 52.158 52.187 0.02020202 1  0.8  1.2
  4:  A 52.187 52.216 0.03030303 1  0.8  1.2
  5:  A 52.216 52.245 0.04040404 1  0.8  1.2
 ---                                        
596:  C  7.755  7.784 0.04040404 3  2.8  3.2
597:  C  7.784  7.813 0.03030303 3  2.8  3.2
598:  C  7.813  7.842 0.02020202 3  2.8  3.2
599:  C  7.842  7.871 0.01010101 3  2.8  3.2
600:  C  7.871  7.900 0.00000000 3  2.8  3.2

绘制结果：

ggplot() +
  geom_rect(data = df.new,
            aes(xmin = xmin, xmax = xmax, 
                ymin = ymin, ymax = ymax, 
                alpha = alpha, fill = zz)) +
  scale_x_continuous(breaks = as.integer(df$zz),
                     labels = df$zz) +
  scale_alpha_identity() +
  theme_bw()