Вот подход с базовым R:
par(mar = c(7,5,3,2))
plot(Snails ~ Distance, data, xlim = c(0, 50), ylim = c(0,30),
pch = 0, xlab = "", ylab = "Snails")
lines(Snails ~ Distance, data)
axis(1, at = c(0,10,20,30,40,50), labels=c(0,data$Age),
line=2.5, col="blue", col.ticks="blue", col.axis="blue")
mtext("Distance (m)", 1, line=1, at=-6)
mtext("Age", 1, line=2.5, at=-6, col = "blue")