Question

I’d like to plot a mirrored 95% density curve and map alpha to the density:

foo <- function(mw, sd, lower, upper) {
x <- seq(lower, upper, length=500)
dens <- dnorm(x, mean=mw, sd=sd, log=TRUE)
dens0 <- dens -min(dens)
return(data.frame(dens0, x))
}

df.rain <- foo(0,1,-1,1)

library(ggplot2)


drf <- ggplot(df.rain, aes(x=x, y=dens0))+
geom_line(aes(alpha=..y..))+
geom_line(aes(x=x, y=-dens0, alpha=-..y..))+
stat_identity(geom="segment", aes(xend=x, yend=0, alpha=..y..))+
stat_identity(geom="segment", aes(x=x, y=-dens0, xend=x, yend=0, alpha=-..y..))
drf

This works fine, but I’d like to make the contrast between the edges and the middle more prominent, i.e., I want the edges to be nearly white and only the middle part to be black. I’ve been tampering with scale_alpha() but without luck. Any ideas?

Edit: Ultimately, I’d like to plot several raindrops, i.e., the individual drops will be small but the shading should still be clearly visible.

Answer 1

While pondering both your answers I actually found exactly what I was looking for. The easiest way is to simply use scale_colour_gradientn with a vector of greys.

library(RColorBrewer)
grey <- brewer.pal(9,"Greys")

drf <- ggplot(df.rain, aes(x=x, y=dens0, col=dens0))+
 stat_identity(geom="segment", aes(xend=x, yend=0))+
 stat_identity(geom="segment", aes(x=x, y=-dens0, xend=x, yend=0))+
 scale_colour_gradientn(colours=grey)
drf