In ggplot, how to make panel.grid.major on top of the barplot?

Question

I wish to produce a graph panel grid lines which are overlaid on top of the figures. (I want to create an effect that is much like what is shown in Tufte).

I created this using geom_segment, but the drawback is that I need to set the heights at which the segments are manually – but I would like that to be set automatically at the height of the ticks.

df <- data.frame(id=1:5, val=abs(rnorm(5)))

ggplot(df, aes(id, val)) + geom_bar(stat="identity") + 
 theme_tufte() + 
 geom_segment(x=-Inf, y=0:4, xend=Inf, yend=0:4, color="white")

Answer 1

You can get the value of the tick break positions from layout$panel_params[[1]]$y.major_source. Then you can just use a horizontal line at those points. For your example it would look something like this:

library(ggthemes)
df <- data.frame(id=1:5, val=abs(rnorm(5)))

myPlot <- ggplot() + geom_bar(data = df, aes(x = id, y = val), stat="identity") + 
  theme_tufte()

myTuftePlot <- ggplot_build(myPlot)
y.ticks <- myTuftePlot$layout$panel_params[[1]]$y.major_source
myPlot + geom_hline(aes(yintercept = y.ticks), colour = 'white')