par(mfrow=c(1,1))

# make labels and margins smaller
par(cex=0.7, mai=c(0.1,0.1,0.2,0.1))

# define area for the plot
par(fig=c(0.1,0.7,0.3,0.9))

# define area for the boxplot
par(fig=c(0.8,1,0,1), new=TRUE)