在ggplot2中的不同等高线图中保持相同的中断

我正在使用 ggplot2 stat_contour_filled 函数创建数据不同子集的等高线图。然而，对于每种颜色，它都会产生不同的色标，这使得很难对它们进行比较。我尝试过使用“breaks=”选项，但由于某种原因它不起作用。 MWE 是：

library(ggplot2)
library(gridExtra)

v <- list()
v[[1]] <- ggplot(faithfuld, aes(waiting, eruptions, z = density)) + stat_contour_filled(breaks=(0:10)*0.005)
v[[2]] <- ggplot(faithfuld, aes(waiting, eruptions, z = 1.1*density)) + stat_contour_filled(breaks=(0:10)*0.005)

of = "mwe.png"
png(of)
print(do.call(grid.arrange,  v))
dev.off()

It produces the following plot.

如何为两个等高线图获得相同的颜色级别？

这里的根本问题是离散尺度会降低空水平。您可以使用drop = FALSE告诉相关秤不要掉空位。

library(ggplot2)

breaks <- (0:10)*0.005

ggplot(faithfuld, aes(waiting, eruptions, z = density)) + 
  stat_contour_filled(breaks = (0:10)*0.005) + 
  scale_fill_viridis_d(drop = FALSE)

ggplot(faithfuld, aes(waiting, eruptions, z = 1.1*density)) + 
  stat_contour_filled(breaks = (0:10)*0.005) +
  scale_fill_viridis_d(drop = FALSE)

^{Created on 2020-11-17 by the reprex package https://reprex.tidyverse.org (v0.3.0)}

或者，您可以明确设置限制。但这有点尴尬，因为它需要将中断值转换为格式化字符串。

library(ggplot2)

make_break_labels <- function(breaks, digits = 3) {
  n <- length(breaks)
  interval_low <- breaks[1:(n-1)]
  interval_high <- breaks[2:n]
  label_low <- format(as.numeric(interval_low), digits = digits, trim = TRUE)
  label_high <- format(as.numeric(interval_high), digits = digits, trim = TRUE)
  sprintf("(%s, %s]", label_low, label_high)
}

breaks <- (0:10)*0.005
break_labels <- make_break_labels(breaks)

ggplot(faithfuld, aes(waiting, eruptions, z = density)) + 
  stat_contour_filled(breaks = (0:10)*0.005) + 
  scale_fill_viridis_d(limits = break_labels)

ggplot(faithfuld, aes(waiting, eruptions, z = 1.1*density)) + 
  stat_contour_filled(breaks = (0:10)*0.005) +
  scale_fill_viridis_d(limits = break_labels)

^{Created on 2020-11-17 by the reprex package https://reprex.tidyverse.org (v0.3.0)}