r - Edit individual ggplots in GGally::ggpairs: How do I have the density plot not filled in ggpairs?

Question

With

library(GGally)

data(diamonds, package="ggplot2")
diamonds.samp <- diamonds[sample(1:dim(diamonds)[1],200),]

# Custom Example
ggpairs(
 diamonds.samp[,1:5],
 mapping = ggplot2::aes(color = cut),
 upper = list(continuous = wrap("density", alpha = 0.5), combo = "box"),
 lower = list(continuous = wrap("points", alpha = 0.3), combo = wrap("dot", alpha = 0.4)),
 diag = list(continuous = wrap("densityDiag")),
 title = "Diamonds"
)

I get

How do I make the diagonal density plots to not be filled, and only show the lines?

Kind of works... but not really.

This is really ugly - in terms of code - because it makes no real sense to me. Also, it does not work here, because it changes the histograms as well.

ggpairs(
  diamonds.samp[,1:5],
  mapping = ggplot2::aes(color = cut),
  upper = list(continuous = wrap("density", alpha = 0.5), combo = "box"),
  lower = list(continuous = wrap("points", alpha = 0.3), combo = wrap("dot", alpha = 0.4)),
  diag = list(continuous = wrap("densityDiag"), mapping = ggplot2::aes(fill=carat)),
  title = "Diamonds"
)

Answer 1

The answer to the question can be found on https://cran.r-project.org/web/packages/GGally/vignettes/ggpairs.html (archived here)

ggally_mysmooth <- function(data, mapping, ...){
  ggplot(data = data, mapping=mapping) +
    geom_density(mapping = aes_string(color="cut"), fill=NA)
}
ggpairs(
  diamonds.samp[,1:5],
  mapping = aes(color = cut),
  upper = list(continuous = wrap("density", alpha = 0.5), combo = "box"),
  lower = list(continuous = wrap("points", alpha = 0.3), combo = wrap("dot", alpha = 0.4)),
  diag = list(continuous = ggally_mysmooth),
  title = "Diamonds"
)