layout(matrix(1:3, ncol = 3))

res <- residuals(cox1)

plot( res ~ age, data = GBSG2, ylim = c(-2.5, 1.5), pch =".", ylab = "Residui di martingala")
abline (h = 0, lty = 3, col = "blue")

plot( res ~ pnodes, data = GBSG2, ylim = c(-2.5, 1.5), pch =".", ylab = "")
abline (h = 0, lty = 3, col = "blue")

plot( res ~ log(progrec), data = GBSG2, ylim = c(-2.5, 1.5), pch =".", ylab = "")
abline (h = 0, lty = 3, col = "blue")