tmpar <- par( mar=c(3.1,3.1,0.5,.6), mfrow=c(1,2) )
on.exit( { par( tmpar ); rm( tmpar ) } )

attach( bactroom )
int.plot( trt, bill, run, bar.plot="none", xlab="", ylab="" )
mtext( "(a) treatment by run", 1, 2 )
mtext( "bill length (mm)", 2, 2 )
detach( )

int.plot( bactroom.bill, bar.plot="none", xlab="", ylab="", lty = c(1,1,2,2) )
mtext( "(b) temp by bact", 1, 3 )