This is how to make the x or y axes only show integer values in a programmatic and scalable fashion.

integer_breaks_ggplot2.R

# A function factory for getting integer y-axis values.
integer_breaks <- function(n = 5, ...) {
    fxn <- function(x) {
        breaks <- floor(pretty(x, n, ...))
        names(breaks) <- attr(breaks, "labels")
        breaks
    }
    return(fxn)
}
        
plot + scale_y_continuous(breaks = integer_breaks())