set.seed(2015-04-13)

d = data.frame(x =seq(1,10),
           n = c(0,0,1,2,3,4,4,5,6,6),
           logp = signif(-log10(runif(10)), 2))

par(mar = c(5,5,2,5))
with(d, plot(x, logp, type="l", col="red3", 
             ylab=expression(-log[10](italic(p))),
             ylim=c(0,3)))
             
par(new = T)
with(d, plot(x, n, pch=16, axes=F, xlab=NA, ylab=NA, cex=1.2))
axis(side = 4)
mtext(side = 4, line = 3, 'Number genes selected')
legend("topleft",
       legend=c(expression(-log[10](italic(p))), "N genes"),
       lty=c(1,0), pch=c(NA, 16), col=c("red3", "black"))