ggridges直方图（stat = binline）下的阴影区域如何？

Question

使用 Dr. Evers suggestion 用 ggridges 对密度曲线下的区域进行着色效果很好。但是，我发现密度曲线可能具有欺骗性，因为它们暗示数据存在，而数据不存在。因此，我想我会尝试使用普通直方图的这种着色技术。

但是，当我尝试将它与直方图一起使用时，阴影有点偏离。这是为什么呢？

library(tidyverse)
install.packages("ggridges", dependencies=TRUE)  # there are many
library(ggridges)
 t2 <-   structure(list(Date = c("1853-01", "1853-02", "1853-03", "1853-04", 
"1853-05", "1853-06", "1853-07", "1853-08", "1853-09", "1853-10", 
"1853-11", "1853-12", "1854-01", "1854-02", "1854-03", "1854-04", 
"1854-05", "1854-06", "1854-07", "1854-08", "1854-09", "1854-10", 
"1854-11", "1854-12"), t = c(-5.6, -5.3, -1.5, 4.9, 9.8, 17.9, 
18.5, 19.9, 14.8, 6.2, 3.1, -4.3, -5.9, -7, -1.3, 4.1, 10, 16.8, 
22, 20, 16.1, 10.1, 1.8, -5.6), year = c("1853", "1853", "1853", 
"1853", "1853", "1853", "1853", "1853", "1853", "1853", "1853", 
"1853", "1854", "1854", "1854", "1854", "1854", "1854", "1854", 
"1854", "1854", "1854", "1854", "1854")), row.names = c(NA, -24L
), class = c("tbl_df", "tbl", "data.frame"), .Names = c("Date", 
"t", "year"))


gg <- ggplot(t2, aes(x = t, y = year)) +
      geom_density_ridges(stat = "binline", bins = 10, scale = 0.8, 
                      draw_baseline = TRUE) +
      theme_ridges()

# Build ggplot and extract data
d <- ggplot_build(gg)$data[[1]]

# Add geom_ribbon for shaded area
gg +
  geom_ribbon(
    data = transform(subset(d, x >= 10), year = group),
    aes(x, ymin = ymin, ymax = ymax, group = group),
    fill = "red",
    alpha = 1.0) 

Answer 1

如果您愿意调整大小并移动 bin 以使 bin 边界恰好位于您的分界线（此处为 10），则以下方法有效。

ggplot(t2, aes(x = t, y = year, fill = ifelse(..x..>=10, ">= 10", "< 10"))) +
  geom_density_ridges_gradient(stat = "binline", binwidth = 3,
                               center = 8.5, scale = 0.8, 
                               draw_baseline = TRUE) +
  theme_ridges() +
  scale_fill_manual(values = c("gray70", "red"), name = NULL)

之所以观察到你所做的效果是因为 x 轴在第一个和第二个绘图之间发生变化，并且 x 轴范围对 bin 的绘制方式有影响。有两种解决方案：您可以固定 x 轴范围或通过 center 和 binwidth 而不是 bins 定义 bin。 （在我看来，无论您如何处理 x 轴，第二个选项总是首选。）

首先，固定x轴范围：

gg <- ggplot(t2, aes(x = t, y = year)) +
  geom_density_ridges(stat = "binline", bins = 10, scale = 0.8, 
                      draw_baseline = TRUE) +
  theme_ridges() +
  scale_x_continuous(limits = c(-12, 28)) # this is where the change is

# Build ggplot and extract data
d <- ggplot_build(gg)$data[[1]]

# Add geom_ribbon for shaded area
gg +
  geom_ribbon(
    data = transform(subset(d, x >= 10), year = group),
    aes(x, ymin = ymin, ymax = ymax, group = group),
    fill = "red",
    alpha = 1.0) 

二、替代bin定义：

gg <- ggplot(t2, aes(x = t, y = year)) +
  geom_density_ridges(stat = "binline",
                      binwidth = 3, center = 8.5, # this is where the change is
                      scale = 0.8, draw_baseline = TRUE) +
  theme_ridges()

# Build ggplot and extract data
d <- ggplot_build(gg)$data[[1]]

# Add geom_ribbon for shaded area
gg +
  geom_ribbon(
    data = transform(subset(d, x >= 10), year = group),
    aes(x, ymin = ymin, ymax = ymax, group = group),
    fill = "red",
    alpha = 1.0) 

Answer 2

确实发生了一些奇怪的事情。请参阅下面的“结论”。

1. 如果我们只绘制gg：

    gg;
   

2. 如果我们绘制gg 加上一个应该对应于gg 轨迹的阶梯：

     gg +
         geom_step(
             data = d, 
             aes(xmax, ymax, group = group), 
             direction = "vh", col = "red",  size = 2);
   

所以添加geom_step 会以某种方式改变gg。我不明白这怎么可能。您可以看到geom_step（红色曲线）实际上与单独绘制gg 时的直方图轨迹相对应（参见第一张图）。