par(mar=c(2, 1, 1, 1)) y1 <- rnorm(100) y2 <- rnorm(100) par(mfrow=c(2, 1), xpd=NA) plot(y1, type="l", axes=FALSE, xlab="", ylab="", main="") box(col="gray") mtext("Left end of margin", adj=0, side=3) lines(x=c(20, 20, 40, 40), y=c(-7, max(y1), max(y1), -7), lwd=3, col="gray") plot(y2, type="l", axes=FALSE, xlab="", ylab="", main="") box(col="gray") mtext("Right end of margin", adj=1, side=3) mtext("Label below x=30", at=30, side=1) lines(x=c(20, 20, 40, 40), y=c(7, min(y2), min(y2), 7), lwd=3, col="gray")