data(cellular)
dev.new()
barplot(100*cells/200 ~ ifn + tnf, beside=TRUE, data=cellular, col=terrain.colors(4),
xlab="Dose of TNF", ylab="% of cells with markers of differentiation")
legend("topleft", legend=c("0","4","20","100"), fill=terrain.colors(4),
title="Dose of IFN", bty="n")
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