R ggplot2::geom_density with a constant variable - r

Question

I have recently came across a problem with ggplot2::geom_density that I am not able to solve. I am trying to visualise a density of some variable and compare it to a constant. To plot the density, I am using the ggplot2::geom_density. The variable for which I am plotting the density, however, happens to be a constant (this time):
df <- data.frame(matrix(1,ncol = 1, nrow = 100))
colnames(df) <- "dummy"
dfV <- data.frame(matrix(5,ncol = 1, nrow = 1))
colnames(dfV) <- "latent"
ggplot() +
geom_density(data = df, aes(x = dummy, colour = 's'),
fill = '#FF6666', alpha = 0.2, position = "identity") +
geom_vline(data = dfV, aes(xintercept = latent, color = 'ls'), size = 2)
This is OK and something I would expect. But, when I shift this distribution to the far right, I get a plot like this:
df <- data.frame(matrix(71,ncol = 1, nrow = 100))
colnames(df) <- "dummy"
dfV <- data.frame(matrix(75,ncol = 1, nrow = 1))
colnames(dfV) <- "latent"
ggplot() +
geom_density(data = df, aes(x = dummy, colour = 's'),
fill = '#FF6666', alpha = 0.2, position = "identity") +
geom_vline(data = dfV, aes(xintercept = latent, color = 'ls'), size = 2)
which probably means that the kernel estimation is still taking 0 as the centre of the distribution (right?).
Is there any way to circumvent this? I would like to see a plot like the one above, only the centre of the kerner density would be in 71 and the vline in 75.
Thanks

Answer 1

Well I am not sure what the code does, but I suspect the geom_density primitive was not designed for a case where the values are all the same, and it is making some assumptions about the distribution that are not what you expect. Here is some code and a plot that sheds some light:
# Generate 10 data sets with 100 constant values from 0 to 90
# and then merge them into a single dataframe
dfs <- list()
for (i in 1:10){
v <- 10*(i-1)
dfs[[i]] <- data.frame(dummy=rep(v,100),facet=v)
}
df <- do.call(rbind,dfs)
# facet plot them
ggplot() +
geom_density(data = df, aes(x = dummy, colour = 's'),
fill = '#FF6666', alpha = 0.5, position = "identity") +
facet_wrap( ~ facet,ncol=5 )
Yielding:
So it is not doing what you thought it was, but it is also probably not doing what you want. You could of course make it "translation-invariant" (almost) by adding some noise like this for example:
set.seed(1234)
noise <- +rnorm(100,0,1e-3)
dfs <- list()
for (i in 1:10){
v <- 10*(i-1)
dfs[[i]] <- data.frame(dummy=rep(v,100)+noise,facet=v)
}
df <- do.call(rbind,dfs)
ggplot() +
geom_density(data = df, aes(x = dummy, colour = 's'),
fill = '#FF6666', alpha = 0.5, position = "identity") +
facet_wrap( ~ facet,ncol=5 )
Yielding:
Note that there is apparently a random component to the geom_density function, and I can't see how to set the seed before each instance, so the estimated density is a bit different each time.