Adding the expectation to a LTT plot

A user writes:

how do I add the a line with the expected number of lineages to a semi-log lineage-through-time plot?

The answer is simple enough, just use the base function lines, for example:

> require(phytools)
> tree<-pbtree(n=100)
> z<-ltt(tree)
> lines(c(0,max(nodeHeights(tree))),c(log(2), log(length(tree$tip))),lty=2)

That's it.