Use distance between y-axis ticks as variable in ggplot2 - r

Question

Is there a way to use the distance between the ticks in the y-axis as variable within ggplot function?
I want to plot an annotation whose y-coordinate can be "responsive" to the y-axis scale of a graph.
In the example below, I would like var1 to be the distance between successive y-axis ticks as taken from the parameters of the specific graph, and not a user-defined constant.
Data:
EXse2<- data.frame(wk=c(1,2), EX=c(4.457143, 2.580952), se=c(0.4209481,0.5519333))
Code:
var1<- 0.1
labEX<- "\u002A\u002A"
library(ggplot2)
ggplot(EXse2, aes(x=factor(wk), y=EX, group=1))+
geom_errorbar(aes(ymin=EX-se, ymax=EX+se), width=.2, size=1)+
geom_line(size=1)+
geom_point(size=2)+
annotate("text", label=labEX, x=1.5, y=(mean(EXse2$EX[1:2])+var1))+
xlab("period")+
ylab(expression(bar(EX)))+
theme(
panel.background=element_blank(),
panel.border = element_blank(),
panel.grid.minor.x=element_blank(),
panel.grid.major.x=element_blank(),
panel.grid.minor.y = element_line(colour = "#d9d9d9"),
panel.grid.major.y = element_line(colour = "#bfbfbf"),
axis.line = element_line()
)

Answer 1

You can achieve this by creating to plot first, extracting the tick distances with ggplot_build and then add the annotation:
library(ggplot2)
EXse2<- data.frame(wk=c(1,2), EX=c(4.457143, 2.580952), se=c(0.4209481,0.5519333))
labEX<- "\u002A\u002A"
#first, put your plot in p1
p1 <-
ggplot(EXse2, aes(x=factor(wk), y=EX, group=1))+
geom_errorbar(aes(ymin=EX-se, ymax=EX+se), width=.2, size=1)+
geom_line(size=1)+
geom_point(size=2)+
# I HAVE REMOVED THE ANNOTATE HERE
xlab("period")+
ylab(expression(bar(EX)))+
theme(
panel.background=element_blank(),
panel.border = element_blank(),
panel.grid.minor.x=element_blank(),
panel.grid.major.x=element_blank(),
panel.grid.minor.y = element_line(colour = "#d9d9d9"),
panel.grid.major.y = element_line(colour = "#bfbfbf"),
axis.line = element_line()
)
# extract axis limits
yminticks <- ggplot_build(p1)$panel$ranges[[1]]$y.minor_source
# subtract first tick from second
var1 <- yminticks[2] - yminticks[1]
# and now add the annotate and plot
p1 +
annotate("text", label=labEX, x=1.5, y=(mean(EXse2$EX[1:2])+var1))