legend at fixed distance form the bottom

Lol.
To me, a "fixed amount" that depends on the size of the graph is not much
fixed...

--- 8< ---

symboles <- c(3,4,5,6)
dn <- rbind(matrix(rnorm(20),,5),matrix(rnorm(20,2),,5))
listSymboles <- rep(symboles,each=2)

layout(matrix(c(1,1,1,2,2,3),3))
for(i in 1:3){
 plot(dn)
 legend("top", pch = unique(listSymboles), legend = c("a",
"b","c","d"),inset = c(0,1.1), horiz = TRUE, xpd = NA)
}

--- 8< ---
On this example, it is easy to see that index is at a fixed distance from
the bottom but that Legend is at a distance "porportional"

I would like to add a legend under a graph but at a fixed distance from
the graphe"

The top of the legend is always a fixed amount below the graph (which is
what you asked for).