Increase number of axis ticks

You can 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()

Gives you this:

https://i.stack.imgur.com/d5jFT.jpg

And overriding the scales can give you something like this:

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))

https://i.stack.imgur.com/brQ5z.jpg

If you want to simply "zoom" in on a specific part of a plot, look at xlim() and ylim() respectively. Good insight can also be found here to understand the other arguments as well.

Based on Daniel Krizian's comment, you can also use the pretty_breaks function from the scales library, which is imported automatically:

ggplot(dat, aes(x,y)) + geom_point() +
scale_x_continuous(breaks = scales::pretty_breaks(n = 10)) +
scale_y_continuous(breaks = scales::pretty_breaks(n = 10))

All you have to do is insert the number of ticks wanted for n.

A slightly less useful solution (since you have to specify the data variable again), you can use the built-in pretty function:

ggplot(dat, aes(x,y)) + geom_point() +
scale_x_continuous(breaks = pretty(dat$x, n = 10)) +
scale_y_continuous(breaks = pretty(dat$y, n = 10))

You can supply a function argument to scale, and ggplot will use that function to calculate the tick locations.

library(ggplot2)
dat <- data.frame(x = rnorm(100), y = rnorm(100))
number_ticks <- function(n) {function(limits) pretty(limits, n)}

ggplot(dat, aes(x,y)) +
  geom_point() +
  scale_x_continuous(breaks=number_ticks(10)) +
  scale_y_continuous(breaks=number_ticks(10))

Starting from v3.3.0, ggplot2 has an option n.breaks to automatically generate breaks for scale_x_continuous and scale_y_continuous

    library(ggplot2)

    plt <- ggplot(mtcars, aes(x = mpg, y = disp)) +
      geom_point()

    plt + 
      scale_x_continuous(n.breaks = 5)

https://i.stack.imgur.com/RUxXg.png

    plt + 
      scale_x_continuous(n.breaks = 10) +
      scale_y_continuous(n.breaks = 10)

https://i.stack.imgur.com/pyTrw.png

Additionally,

ggplot(dat, aes(x,y)) +
geom_point() +
scale_x_continuous(breaks = seq(min(dat$x), max(dat$x), by = 0.05))

Works for binned or discrete scaled x-axis data (I.e., rounding not necessary).

A reply to this question and How set labels on the X and Y axises by equal intervals in R ggplot?

mtcars %>% 
  ggplot(aes(mpg, disp)) +
  geom_point() +
  geom_smooth() + 
  scale_y_continuous(limits = c(0, 500),
                     breaks = seq(0,500,50))  +
  scale_x_continuous(limits = c(0,40),
                     breaks = seq(0,40,5))