tmp <- par( mar = c(3.1,3.1,0,0), mfrow = c(1,2) )
on.exit( par( tmpar ) )

mplot( jitter( predict( growth.fit ) ), jitter( resid( growth.fit ) ),
   group = growth$code, xlab = "", ylab = "" )
mtext( "(a) mean with zeroes", 1, 2 )
mtext( "residuals", 2, 2 )
abline( h = 0, lty = 2 )
abline( h = c(-1,1) * std.dev( growth.fit ), lty = 3 )
abline( 0, -1 )

mplot( jitter( predict( growth.zero.fit ) ), jitter( resid( growth.zero.fit ) ),
   group = growth.zero$code, xlab = "", ylab = "" )
mtext( "(b) mean without zeroes", 1, 2 )
abline( h = 0, lty = 2 )
abline( h = c(-1,1) * std.dev( growth.zero.fit ), lty = 3 )
abline( 0, -1 )

par( tmp )