Draw segment below density plot using relative y?

Question

I want to draw a segment below a density plot and I would like to have the distance be a constant number of pixels. Is this possible? I Know how to hardcode the distance. For example:

set.seed(40816)
library(ggplot2)
df.plot <- data.frame(x = rnorm(100, 0, 1))
ggplot(df.plot, aes(x = x)) + geom_density() +
  geom_segment(aes(x = -1, y = -0.05, xend = 1, yend = -0.05),
               linetype = "longdash")

produces :

But

df.plot <- data.frame(x = rnorm(100, 0, 4))
ggplot(df.plot, aes(x = x)) + geom_density() +
  geom_segment(aes(x = -1, y = -0.025, xend = 1, yend = -0.025),
               linetype = "longdash")

produces a plot with the segment much further away from the density

Answer 1

You could use annotation_grob,

set.seed(40816)
library(ggplot2)
df.plot <- data.frame(x = rnorm(100, 0, 1))
strainerGrob <- function(pos=unit(2,"mm"), gp=gpar(lty=2, lwd=2))
  segmentsGrob(0, unit(1,"npc") - pos, 1, unit(1,"npc") - pos, gp=gp)

ggplot(df.plot, aes(x = x)) + geom_density() +
  annotation_custom(strainerGrob(), xmin = -1, xmax = 1, ymin=-Inf, ymax=0) +
  expand_limits(y=-0.1)