Two plot with a common legend - base graphics

If you need to share a common legend between two graphs using the ggplot2 package/paradigm take a look at this post from the Learning R blog.
The code below solves the same task using the R base graphics.

png( "2plot1legend.png", width = 480, height = 680)
par(mfrow = c(2, 1), oma = c(0, 0, 0, 2))
plot(hp~mpg, data=mtcars, col=cyl,pch=19)
plot(disp~wt, data=mtcars, col=cyl,pch=19)
par(xpd=NA)
#legend(locator(1), legend=as.numeric(levels(factor(mtcars$cyl))), pch=19, col= as.numeric(levels(factor(mtcars$cyl))) )
legend(x=5.6, y=690, legend=as.numeric(levels(factor(mtcars$cyl))), pch=19, col= as.numeric(levels(factor(mtcars$cyl))) )
dev.off()