Hi, I have followed this paper but I have problem to undestand the following piece of code:

> pch <- c(0,1,2,15,16,17)
> colors <- rep(c("darkgreen", "red", "blue"), 2)
> plotMDS(y, col=colors[group], pch=pch[group])
> legend("topleft", legend=levels(group), pch=pch, col=colors, ncol=2)

What do these numbers mean c(0,1,2,15,16,17)?

Thank you in advance.

Have a look at ?points. Or, should you need something more visual, try:

plot(1:25, pch=1:25)
text(1:25, 1:25, pos=3)