How to get shaded background in xyplot in R?

Question

using xyplot from the lattice package, I plot a time series over a number of years. I would to add a shaded area for some of these years to indicate that this time period was "special" (e.g. war).

Please apologize if this is trivial, but I could not figure out how to do that, so I would be happy if someone could help me out, or at least point me in the right direction. I think my main problem is that I don't really know how to approach this problem. I am still relatively new to R, and to lattice in particular.

Here a minimal example:

 xyplot( rnorm(100) ~ 1:100, type="l", col="black")

In the corresponding plot, I would like the color of the background (from say x-values of 45 until 65) from the bottom to the top of the plotting area be shaded in, say, light grey.

Note that solutions that I have found so far use base graphics and the polygon-function, but there the intention is to shade the area under or above a curve, which is different from what I would like to do. I don't "just" want to shade the area below my line, or above my line. Instead I would like to shade the entire background for a given time interval.

If anyone could help me out here, I would be very grateful!

Answer 1

See ?panel.xblocks in the latticeExtra package:

library(latticeExtra)
x <- 1:100
xyplot( rnorm(100) ~ x, type="l", col="black") + 
         layer_(panel.xblocks(x, x > 20, col = "lightgrey"))

Answer 2

Try this:

 xyplot( 
    rnorm(100) ~ 1:100, type="l", col="black",
    panel=function (x,y,...){
    panel.rect(xleft=45, xright=65,ybottom=-3, ytop=3,col="grey")
    panel.xyplot(x,y,...)
    }
  )

The panel.rect() function controls the rectangle and is the lattice equivalent of the rect() function. It has a variety of settings that you may find useful. It is called first and then the xyplot() is put on top of it. You many need to play around a little to get your ybottom and ytop parameters to look as you like them.

Answer 3

trellis.focus("panel", 1, 1)
grid.rect(x =.55, , y=.5, w = .2, height=6,  
  gp = gpar(fill = "light grey"))
trellis.unfocus()

This differs from @JohnPaul's solution in a couple of ways (and I think his answer is better). This uses the center of the desired X-band for placement in "native coordinates" and calculates the width as 'range(xlim)/range(band)' and it modifies an existing plot. the grid.rect function is the grid packages lower level function that is used by panel.rect. I sometimes find this useful when integrating lattice panels inside the xyplot system defeats me.