How to add box layer using levelplot in R?

Question

I would like to add some boxes with special extent to my plot.

Example

     gh <- raster()
     gh[] <- 1:ncell(gh)
     SP <- spsample(Spatial(bbox=bbox(gh)), 10, type="random")

Then plot them

  levelplot(gh, col.regions = rev(terrain.colors(255)), cuts=254, margin=FALSE) +
  layer(sp.points(SP, col = "red"))

this plots a map with several crosses in it but I need to plot a box with spacial extent:

     extent(gh) = extent(c(xmn=-180,xmx=180,ymn=-90,ymx=90))
      e6 <- extent( 2  , 8 , 45   , 51  )

I wan to add e6 to the plot and put the number2 inside the box.Any hint please

Answer 1

Convert the Extent object into a SpatialPolygons, and extract its centroid with coordinates:

library("raster")
library("sp")
library("rasterVis")

gh <- raster()
gh[] <- 1:ncell(gh)
SP <- spsample(Spatial(bbox=bbox(gh)), 10, type="random")

e6 <- extent( 2, 8, 45, 51)
e6pol <- as(e6, 'SpatialPolygons')
centroid <- coordinates(e6pol)

levelplot(gh, col.regions = rev(terrain.colors(255)), cuts=254, margin=FALSE) +
    layer({sp.points(SP, col = "red")
           sp.polygons(e6pol)
           panel.text(centroid[,1], centroid[,2], '2')
           })