[R] How to plot a matrix

[ss]

Hi list,

If I have a matrix,

> a<-read.table("a.txt",header=TRUE,row.names=1)
> dim(a)
[1] 4 3
> a
   x y z
a1 1 6 3
a2 2 6 3
a3 2 3 8
a4 5 3 4
>

Can you suggest how to produce a plot like the one attached here? Basically,
I want to plot each sample (each column) using the dots one by one and also
have the average value of each column indicated?

Thank you so much and have a wonderful weekend!

Allen