r - Adding vertical line in plot ggplot

Question

I am plotting a graph using the following piece of code:

library (ggplot2)

png (filename = "graph.png")
stats <- read.table("processed-r.dat", header=T, sep=",")
attach (stats)
stats <- stats[order(best), ]
sp <- stats$A / stats$B
index <- seq (1, sum (sp >= 1.0))
stats <- data.frame (x=index, y=sp[sp>=1.0])
ggplot (data=stats, aes (x=x, y=y, group=1)) + geom_line()
dev.off ()

1 - How one can add a vertical line in the plot which intersects at a particular value of y (for example 2)?

2 - How one can make the y-axis start at 0.5 instead of 1?

Answer 1

You can add vertical line with geom_vline(). In your case:

+ geom_vline(xintercept=2)

If you want to see also number 0.5 on your y axis, add scale_y_continuous() and set limits= and breaks=

+ scale_y_continuous(breaks=c(0.5,1,2,3,4,5),limits=c(0.5,6))