Gradient fill in ggplot2

Question

Say if have the following plot.

library(ggplot2)
n <- 1169
df22 <- data.frame(x = 1:n, val = seq(0, 0.5, length.out = n), type = 1)

ggplot(df22, aes(x = x, y = val)) +
  geom_ribbon(aes(ymax = val, ymin = 0, fill = type, group = type))

Instead of the blue color i would like to have a Gradient fill (from blue to red - vertically. So starting with blue at the bottom and red on top with a Parameter to Control the smoothness of Color change).

I found the following resource: https://ggplot2.tidyverse.org/reference/scale_gradient.html

Unfortunately, it didnt work out for me as my data is not continous(?).

Answer 1

The following code will do it (but horizontally):

library(scales) # for muted
ggplot(df22, aes(x = x, y = val)) +
  geom_ribbon(aes(ymax = val, ymin = 0, group = type)) +
  geom_col(aes(fill = val)) +
  scale_fill_gradient2(position="bottom" , low = "blue", mid = muted("blue"), high = "red", 
                       midpoint = median(df22$val)) 

If you want to make it vertically, you may flip the coordinates using coord_flip() upside down.

ggplot(df22, aes(x = val, y = x)) +
  geom_ribbon(aes(ymax = val, ymin = 0)) +
  coord_flip() +
  geom_col(aes(fill = val)) +
  scale_fill_gradient2(position="bottom" , low = "blue", mid = muted("blue"), high = "red", 
                       midpoint = median(df22$val)) 

Or, if you want it to be horizontal with a vertical gradient (as you requested), you might need to go around it by playing with your data and using the geom_segment() instead of geom_ribbon(), like the following:

vals <- lapply(df22$val, function(y) seq(0, y, by = 0.001))
y <- unlist(vals)
mid <- rep(df22$x, lengths(vals))
d2 <- data.frame(x = mid - 1, xend = mid + 1, y = y, yend = y)

ggplot(data = d2, aes(x = x, xend = xend, y = y, yend = yend, color = y)) +
  geom_segment(size = 1) +
  scale_color_gradient2(low = "blue", mid = muted("blue"), high = "red", midpoint = median(d2$y)) 

This will give you the following:

Hope you find it helpful.

Answer 2

This can now more easily be achieved using ggplot2 >= 3.5.0 (and R >= 4.2.0) which adds support for gradient fills.

The first step is to specify a gradient fill, for example using grid::linearGradient where the direction of the gradient can be set via the x1, x2, y1 and y2 arguments. In your case, for example, we have to deviate from the defaults by setting x2 = unit(0, "npc") to get a gradient in the vertical direction.

This gradient can then be applied to the geom by passing it to the fill= argument:

library(ggplot2)
library(grid)

grad_ungroup <- linearGradient(
  c("blue", "red"),
  x1 = unit(0, "npc"), y1 = unit(0, "npc"),
  x2 = unit(0, "npc"), y2 = unit(1, "npc")
)

ggplot(df22, aes(x = x)) +
  geom_ribbon(aes(ymax = val, ymin = 0),
    fill = grad_ungroup
  )