R help - Apr 2014 - Setting alternative x-axis breaks using gglpot2

I'm not doing a Manhattan plot, but plotting AD (coloured by DP) along
the genome:

 points <- ggplot(sam,aes(x = midpoint,y = ad, colour = dp, size = 3)) +
  geom_point() +
  scale_y_continuous(breaks=c(0,20,30,40)) +
  labs(x = "chr",y = "ad") +
  scale_colour_gradient2(high="red", mid="green")

However, instead of having the BP position along the bottom, I was
wondering whether its possible to have the chromosome instead.  Is
there an easier way to do this?

I'm also trying to reduce the size of the points on the
"Manhattan"
plot but changing the size in the code does not work.

Thanks!