You can try to override ggplots default scales by modifying scale_x_continuous and/or scale_y_continuous.

For example:

library(ggplot2)

dat <- data.frame(x = rnorm(100), y = rnorm(100))

ggplot(dat, aes(x,y)) + geom_point()

And overriding the scales can be done as follows:

ggplot(dat, aes(x,y)) +

geom_point() +

scale_x_continuous(breaks = round(seq(min(dat$x), max(dat$x), by = 0.5),1)) +

scale_y_continuous(breaks = round(seq(min(dat$y), max(dat$y), by = 0.5),1))