Provide shades between dates on x axis [duplicate]

Answer 1

Make a data.frame with columns that are 3 and 6 pm for each day covering the data. Use that for a geom_rect layer (using Inf and -Inf for the y's). Put that layer before your data layers (so it is below them) and give the fill an alpha so the grid can be seen through it.

More detailed answer would be possible with a more detailed, reproducible question.

Answer 2

More or less following what Brian Diggs suggests above,

#sample data
set.seed(666)
dat <- data.frame(x = seq(as.POSIXct('2011-03-27 00:00:00'), 
len= (n=24), by="1 hour"), y = cumsum(rnorm(n)))
#Breaks for background rectangles
rects <- data.frame(xstart = as.POSIXct('2011-03-27 15:00:00'), 
xend = as.POSIXct('2011-03-27 18:00:00'))

library(ggplot2)
ggplot() + 
  geom_rect(data = rects, aes(xmin = xstart, xmax = xend, 
            ymin = -Inf, ymax = Inf), alpha = 0.4) + 
  geom_line(data = dat, aes(x,y))

Would give you this,