require(rgeos)

grid_lines = function(x, y, crs='+proj=longlat +datum=WGS84'){
  x1 = round(seq(min(x),max(x),length.out=50),3)
  y1 = round(seq(min(y),max(y),length.out=50),3)
  str = character(0)
  for(i in x) for(j in 2:length(y1)) str = c(str, paste(i, y1[j], ',', i, y1[j-1]))
  for(i in y) for(j in 2:length(x1)) str = c(str, paste(x1[j], i, ',', x1[j-1], i))
  grid = readWKT(paste('MULTILINESTRING((', paste(str, collapse='),('), '))'))
  grid@proj4string = CRS(crs)
  grid
}

## Example
# g = grid_lines(10*(-3:3), 10*(3:7))