[R] Clipping of display in Lattice graphics

[Matthew Walker]

I'm baffled as to how the Lattice package achieves clipping.  Would 
someone mind explaining this to me?

Firstly, my attempt using "just" the grid package:

x<-seq(0,3,by=0.3)/2.8
y<-seq(0,1,by=0.1)

grid.newpage()
grid.rect(gp=gpar(fill="pink"))

vp<-viewport(width=0.8, height=0.8)
pushViewport(vp)

grid.rect(gp=gpar(fill="white"))
grid.xaxis()
grid.yaxis()

grid.points(x,y)
grid.line(x,y)

If you run this you will see a datapoint and line extend into the pink area.

However, if xyplot is used, it doesn't extend past its borders:

xyplot(y~x, xlim=c(0,1), ylim=c(0,1), type=c("p","l"))

 From what I understand so far:
xyplot calls panel.xyplot
panel.xyplot calls lpoints and llines
neither lpoints nor llines do much except call xy.coords and then call 
lplot.xy
lplot.xy calls grid.points and grid.lines

Where in this code does the clipping occur?  I'm quite baffled really.

Thank you for considering this,

Matthew