Júlio,

Your code is not reproducible, you doesn't provide any data. So I did a
minimal code that illustrates a possible procedure is the following

n <- 30
da <- data.frame(x=runif(n), y=runif(n), z=runif(n))
da$z <- cut(da$z, seq(0,1,0.25))

require(lattice)
xyplot(y~x, da, cex=as.numeric(da$z), col=1,
       key=list(points=list(cex=1:4, pch=1), text=list(levels(da$z)),
columns=4))

Bests,
Walmes.

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Walmes Marques Zeviani
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Departamento de Estatística - Universidade Federal do Paraná
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