: put grids in a plot at specific x points

Hi dear all!

I wonder if anybody can help me with this:

I have a plot:
 allchr <- read.table("allchrtog.txt", header=F)

attach(allchr)
names(allchr)

[1] "V1" "V2" "V3" "V4"

plot(V1,V3, type="n")
lines(V1,V3)

and I want to add grids to the plot but just at specific x points. So
for example, the first would be at 286.16 in the x axis.
Does anybody know how can I add these lines?

Thank you so much!

Laura