R - draw new layer behind current plot - r

Question

Just curious, when plotting in R, one can easily change the order of the executive code to change the order of those "layer" on the plot, e.g.
plot(x, type = "n")
lines(y)
points(x)
to get x over the y. Are there any way to do it in an adhoc way, e.g.
plot(x)
lines(y, behind = TRUE) # fictional option behind

Answer 1

While there isn't explicitly a behind option or layers in plot, an easy way to overlay two plots might be using the add = TRUE option in plot. Here is an example with artificial data:
# Load sp package for creating artificial data
library(sp)
# Create sample town points
towns <- data.frame(lon = sample(100), lat = sample(100))
towns <- SpatialPoints(towns)
# Create sample polygon grid
grd <- GridTopology(c(1,1), c(10,10), c(10,10))
polys <- as.SpatialPolygons.GridTopology(grd)
# Plot polygons
plot(polys)
# Add towns (in red colour)
plot(towns, add = TRUE, col = 'red')
As another example, you can plot lines on different layers in ggplot and melt like this:
a <- c(3, 6, 16, 17, 11, 21)
b <- c(0.3, 2.3, 9, 9, 5 ,12)
c <- c(3, 7, 9, 7, 6, 10)
dat <- data.frame(a=a,b=b,c=c)
dat <- melt(dat)
Add an explicit 'x' variable to our data frame:
dat$x <- rep(1:6,times=3)
Then just plot the graph:
ggplot(dat,aes(x=x,y=value)) +
geom_line(aes(colour=variable)) +
scale_colour_manual(values=colours) +
labs(x="time[h]",y="a",colour="") +
opts(title="bla")
Finally, there is explicit support for layers in other packages, such as in PBSmapping for maps.