Set alpha and remove black outline of density plots in ggpairs

Question

Consider this example:

data(tips, package = "reshape")
library(GGally)
pm <- ggpairs(tips, mapping = aes(color = sex), columns = c("total_bill", "time", "tip"))
pm

How do I make the density plots more transparent and remove the black lines?

The GGally packages seems to have changed a lot recently and I cannot find a working solution

update

I found how to change the alpha with a custom function:

my_dens <- function(data, mapping, ..., low = "#132B43", high = "#56B1F7") {
  ggplot(data = data, mapping=mapping) +
    geom_density(..., alpha=0.7) 
}

pm <- ggpairs(tips, mapping = aes(color = sex), columns = c("total_bill", "time", "tip"),
              diag=list(continuous=my_dens))
pm

but the black line still remains.

Answer 1

thanks to @Henrik this is the solution using a custom function

my_dens <- function(data, mapping, ...) {
  ggplot(data = data, mapping=mapping) +
    geom_density(..., alpha = 0.7, color = NA) 
}

pm <- ggpairs(tips, mapping = aes(color = sex), columns = c("total_bill", "time", "tip"),
              diag = list(continuous = my_dens))
pm

Examples on how to customize ggpairs plots can be found in the vignette. See the "Matrix Sections" and "Plot Matrix Subsetting".