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()
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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))
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