[R] plotting box plots on same x

[BJ]

I am trying to construct a graph of 6 box plots of blood pressures. I 
want them to be on a single set of axis and I want the SBP to be ontop 
of the DBP. I have an array bp with the data in it and I tried

a[1,]<-c(145,60,147,62,140,57)
a[2,]<-c(160,75,160,74,160,70)
a[3,]<-c(140,55,140,65,142,55)
 boxplot(data.frame(a), main = "Blood Pressures", at=c(1,1,2,2,3,3), 
names=c("sit","","lie","","stand",""))

which is close to what I want, but it gives me a bunch of empty space at 
the end. is there a better way to do this to avoid this?

As always, Thank You. ~Erithid