x axis errors in chapter 1 histograms

[tomlindsey]

R 2.15 on OS X gglpot2 0.9.0 scales 0.2.0

01-Using_R/code/ufo_sightings.R

I wonder if

scale_x_date(major = "50 years")

should be something like

scale_x_date(breaks = date_breaks("50 years"), labels = date_format("%Y"))