How to add a gradient fill to a geom_density chart - r

Question

I have a dataset where I'd like to plot the density of one column and add a gradient fill that is associated with another column.
For example, this code creates the following plot
library(datasets)
library(tidyverse)
df <- airquality
df %>%
group_by(Temp) %>%
mutate(count = n(),
avgWind = mean(Wind)) %>%
ggplot(aes(x = Temp, fill = avgWind)) +
geom_density()
What I'd like is for the plot to have a gradient fill that indicates what the average wind (avgWind) was at each temperature along the x-axis.
I've seen some examples that allow me to create a gradient fill that is associated with the values on the x-axis itself (in this case, Temp) or by percentile/quantiles, but I'd like the gradient fill to be associated with an additional variable.
It's sort of like this, but instead of a bar plot, I'd like to keep it as a smoothed density chart:
df %>%
group_by(Temp) %>%
mutate(count = n(),
avgWind = mean(Wind)) %>%
ggplot(aes(x = (Temp), fill = avgWind, group = Temp)) +
geom_bar(aes(y = (..count..)/sum(..count..)))

Answer 1

You can't do gradient fills in geom_polygon so the usual solution is to draw lots of line segments. For example you could do something like this:
library("datasets")
library("tidyverse")
library("viridis")
df <- airquality
df <- df %>%
group_by(Temp) %>%
mutate(count = n(), avgWind = mean(Wind))
## Since we (presumably) want continuous fill, we need to interpolate to
## get avgWind at each Temp value.
## The edges are grey because KDE is estimating density
## Where we don't know the relationship between temp and avgWind
d2fun <- approxfun(df$Temp, df$avgWind)
#> Warning in regularize.values(x, y, ties, missing(ties)): collapsing to unique
#> 'x' values
dens <- density(df$Temp)
dens_df <- data.frame(x = dens$x, y = dens$y, fill = d2fun(dens[["x"]]))
ggplot(dens_df) +
geom_segment(aes(x = x, xend = x, y = 0, yend = y, color = fill)) +
scale_color_viridis()