"hist"(x,breaks=NA,main="",xlab="Score",ylab="Frequency", xlim=NA,ylim=NA,col=2:3,border=par("fg"),only.pairs=FALSE,...)
The default is to display all pre and post scores. This may lead to plots that look wrong because the measures of central tendency were calculated for pairs rather than all scores. Setting only.pairs will display only the pairs of scores.