Is there a way to limit vline lengths in ggplot2

Question

I'm trying to show an intercept on a line graph using the ggplot vline and hline but want the lines to cease at the point of interception on the graph. Is this possible either in ggplot or is there another solution

library(ggplot2)

pshare <- data.frame()

for (i in 1:365) {
  pshare <- rbind(pshare,c(i, pbirthday(i,365,coincident=3)))
}

names(pshare) <- c("number","probability")

x25 <- qbirthday(prob = 0.25, classes = 365, coincident = 3) #61
x50 <- qbirthday(prob = 0.50, classes = 365, coincident = 3)
x75 <- qbirthday(prob = 0.75, classes = 365, coincident = 3)

p <- qplot(number,probability,data=subset(pshare,probability<0.99))

p <- p + geom_vline(xintercept = c(x25,x50,x75))
p <- p + geom_hline(yintercept = c(0.25,0.5,0.75))
p

So, for example, I would like the 0.25/61 lines to end when they meet on the plot

TIA

Answer 1

Expanding the comment by @joran into an answer and example

geom_vline draws the whole way across the plot; that is its purpose. geom_segment will only draw between specific end points. It helps to make a data frame with the relevant information for drawing the lines.

probs <- c(0.25, 0.50, 0.75)
marks <- data.frame(probability = probs,
                    number = sapply(probs, qbirthday, classes=365, coincident=3))

With this, making the lines only go to the intersection is easier.

qplot(number,probability,data=subset(pshare,probability<0.99)) +
  geom_segment(data=marks, aes(xend=-Inf, yend=probability)) +
  geom_segment(data=marks, aes(xend=number, yend=-Inf))