# Setup

x1 <- rnorm(100)
x2 <- rnorm(100, mean=5)

# Graphics
hist(c(x1,x2))

xrange<-range(x1,x2)
h1 <- hist(x1, breaks=(seq(floor(xrange[1]), ceiling(xrange[2]))), plot=F)
h2 <- hist(x2, breaks=(seq(floor(xrange[1]), ceiling(xrange[2]))), plot=F)
yrange <- c(0,range(h1$counts,h2$counts)[2])
plot(h1, xlim=xrange, ylim=yrange, col="blue")
par(new=T)
plot(h2, xlim=xrange, ylim=yrange)
par(new=F)

opar <- par(mfrow=c(2,2))
plot(h1); plot(h2)
plot(h1, xlim=xrange, ylim=yrange, col="blue")
plot(h2, xlim=xrange, ylim=yrange)
par(opar)

# Lattice
dfx1 <- data.frame(x=x1, source="x1")
dfx2 <- data.frame(x=x2, source="x2")
df   <- rbind(dfx1, dfx2)

library(lattice)
histogram(~x, data=df)
histogram( ~x, data=df, layout=c(1,2))
histogram(~x | source, data=df, scales=list(alternating=FALSE))