tmpar <- par( mar=c(3.1,3.1,0,.5), mfrow=c(1,2) )
on.exit( { par( tmpar ); rm( tmpar ) } )

attach( bactroom )

lsd.plot( temp, bill, bact, ylim=range( bill ), xlab="", ylab="", ypos=40,
   lty = 1 )
mtext( "(a) interaction plot", 1, 2)
mtext( "bill length (mm)", 2, 2)

par(pty="s")
margin.plot( temp, bill, bact, ylim=range( bill ), effects=T, xlab="", ylab="",
   ypos=40, lty = 1 )
mtext( "(b) margin plot", 1, 2)
abline(0,1,lty=3)
par(pty="m")

detach( )