Admixture Plot Problem

Question

[mscpythonstudy]

I would like to put the sample name on the axis e.g Sample 1 - Sample 19. May I know how to add that inform on the x-axis of the Admixture plot? How I can I know which bar is corresponding to which sample?Thanks in advance.

Here is my code

pdf("structure.pdf")
tbl=read.table("plink.3.Q")
barplot(t(as.matrix(tbl)), col=rainbow(3), xlab="Individual #", ylab="Ancestry", border=NA)
dev.off()

This the plot