Creating two continuous color gradient scales for the same variable in ggplot2

Question

I have a dataframe containing two variables Type and Time, and I am attempting to visualize the values from the Value column using geom_tile. The tiles are currently colored according to a continuous color gradient, ranging from red (indicating high values) to white (indicating low values). However, I would like to apply separate color gradients for the Type variable based on the Category it belongs to. Specifically, I want to use a navy-to-white gradient (with navy indicating high values) for the BB category and a red-to-white gradient (with red indicating high values) for the SS category.

Here's what I have tried so far:

library(ggplot)


df <- data.frame(Type = c(rep("A", 5), rep("B", 5), rep("C", 5)),
                 Time = rep(1:5, 3),
                 Value = round(runif(15, min = -3, max = 3), 2),
                 Category = c(rep("BB", 5), rep("SS", 10)))


ggplot(data = df, 
   aes(x = Time, 
       y = Type, 
       fill = Value)) + 
geom_tile(height = 0.8, 
            width = 1) +
  scale_fill_gradient2(limits = c(-3, 3), 
                       breaks = c(-3, 0, 3),
                       low = "white",
                       mid = "mistyrose",
                       high = "#A63446",
                       midpoint = 0,
                       guide = guide_colorbar(frame.colour = "black", 
                                              ticks.colour = "black")) +
  theme_bw()

Answer 1

You can achieve your desired result easily using the ggnewscale package which allows for multiple scales for the same aesthetic:

library(ggplot2)
library(ggnewscale)

ggplot(
  data = df,
  aes(
    x = Time,
    y = Type,
    fill = Value
  )
) +
  geom_tile(
    data = ~subset(.x, Category == "SS"),
    height = 0.8,
    width = 1
  ) +
  scale_fill_gradient2(
    limits = c(-3, 3),
    breaks = c(-3, 0, 3),
    low = "white",
    mid = "mistyrose",
    high = "#A63446",
    midpoint = 0,
    guide = guide_colorbar(
      frame.colour = "black",
      ticks.colour = "black"
    ),
    name = "SS"
  ) +
  ggnewscale::new_scale_fill() +
  geom_tile(
    data = ~subset(.x, Category == "BB"),
    aes(
      x = Time,
      y = Type,
      fill = Value
    ),
    height = 0.8,
    width = 1
  ) +
  scale_fill_gradient(
    limits = c(-3, 3),
    breaks = c(-3, 0, 3),
    low = "white",
    high = "navy",
    guide = guide_colorbar(
      frame.colour = "black",
      ticks.colour = "black"
    ),
    name = "BB"
  ) +
  theme_bw() +
  theme(
    legend.box = "horizontal"
  )