How to make ggplot2 plots prettier?

Question

I have generated the following plot using the R code that follows it:

ggplot(lengths, aes(length, fill = library)) + geom_density(alpha = 0.2) + coord_cartesian(xlim = c(0, 60000)) 

Now I would like to make the plot a bit prettier:

1. Make the x-axis show length every 5000 units (instead of every 20000)
2. Add x-values on top of the three peaks (approx 3000,5000 and 35000).

How can I do that?

update in response to James:

Answer 1

How about:

(first create a reproducible example)

set.seed(1001)
lengths <- data.frame(length=c(rgamma(1000,shape=10,scale=500),
                    10000+rgamma(1000,shape=5,scale=700),
                    rnorm(500,mean=30000,sd=2000)),
                  library=factor(rep(2:1,c(2000,500))))

(cute stuff to find peak locations and heights)

peakfun <- function(x) {
  d <- density(x$length)
  peaks <- which(diff(sign(diff(d$y)))==-2)
  data.frame(x=d$x[peaks],y=d$y[peaks])
}

peakdat <- ddply(lengths,.(library),peakfun)
peakdat <- peakdat[-1,] ## drop spurious peak

(draw the plot)

library(ggplot2)
ggplot(lengths, aes(length, fill = library)) +
  geom_density(alpha = 0.2) +
  scale_x_continuous(limits = c(0,60000),
                     breaks = seq(0,60000,by=5000))+
  geom_text(data=peakdat,aes(x=x,y=y,label=round(x)),vjust=1)

you probably want to tweak the vertical height of the labels a little