Align grid with ticks

Question

When I let plot draw the axis, grid aligns with ticks by default.

However, my plot is somewhat involved:

mytime <- as.POSIXct("2015-08-20") + seq(0,by=3600,length.out=7*24)
plot(x=mytime,y=rnorm(7*24),xaxt="n")
ticks <- mytime[seq(1,by=12,to=length(mytime))]
axis(1, ticks, strftime(ticks, "%a %H:%M"))
grid(ny=NULL,nx=14)

and I cannot get the grid to align with the ticks:

How do I make them align?

Jamijamie

Answer 1

Looks like abline is the answer:

mytime <- as.POSIXct("2015-08-20") + seq(0,by=3600,length.out=7*24)
plot(x=mytime,y=rnorm(7*24),xaxt="n")
ticks <- c(mytime[seq(1,by=12,to=length(mytime))],mytime[1]+7*24*3600)
axis(1, ticks, strftime(ticks, "%a %H:%M"))
grid(ny=NULL,nx=NA)
abline(v=ticks[seq(1,length(ticks),2)],lty="dotted",col="lightgray")

Jamijamie

Answer 2

As pointed in the previous answer, the solution is using the abline function and avoid that the vertical guidelines are plotted in wrong locations. You can achieve both things by replacing the last line of your code

grid(ny=NULL,nx=14)

by

grid(ny=NULL,nx=NA)
abline(v = ticks, lty = 3, col = "lightgray")

to obtain

By the way, there are previous questions related to this that could have helped you:

1. R: Finding the location of tick marks when plotting dates
2. Align grid() to plot ticks

Scoggins