Convenience function to create 'nice' breaks with ggplot2

annual_breaks.R

## create 'nice' annual breaks for use with ggplot
## because sometimes it's tedious to 'manually' set the minimum and maximum of the date range, 
## when the data starts on odd years, like 1927, but you really want the labels to start at 1925
## x denotes the date variable, e.g. x = df$date
## for m = 5, the breaks will be multiples of 5 years
## breaks below min and above max
## replace floor and ceiling with round, if desired

annual_breaks <- function(x, m = 5) {
    require('scales')
    seq(as.Date(paste0(m*floor(as.numeric(format(min(x), "%Y"))/m), "-01-01")),
    as.Date(paste0(m*ceiling(as.numeric(format(max(x), "%Y"))/m), "-01-01")), 
    by = paste0(m, " years"))
}