tmpar <- par( mar = c(3,3,0,0) )
on.exit( { par( tmpar ); rm( tmpar ) } )

attach( diet )

tmp <- !is.na(covar)

mplot( covar[tmp], dmi[tmp], trt[tmp],
   xlab = "", ylab = "" )
mtext( "covariate = first 3 weeks", 1, 2 )
mtext( "dry matter intake (dmi)", 2, 2 )
mlines( covar[tmp], predict( diet.cov ), trt[tmp] )
legend( min( covar[tmp] ), max( dmi[tmp] ), levels( trt ),
   lty = seq( levels( trt ) ) )
detach( )
se.bar( 26, 20, std.dev( diet.cov ), cap = "SD" )