The Cartesian coordinate system is the most familiar, and common, type of coordinate system. Setting limits on the coordinate system will zoom the plot (like you're looking at it with a magnifying glass), and will not change the underlying data like setting limits on a scale will.
Examples
# There are two ways of zooming the plot display: with scales or
# with coordinate systems. They work in two rather different ways.
p <- ggplot(mtcars, aes(disp, wt)) +
geom_point() +
geom_smooth()
p
# Setting the limits on a scale converts all values outside the range to NA.
p + scale_x_continuous(limits = c(325, 500))
#> Warning: Removed 24 rows containing non-finite values (stat_smooth).
#> Warning: Removed 24 rows containing missing values (geom_point).
# Setting the limits on the coordinate system performs a visual zoom.
# The data is unchanged, and we just view a small portion of the original
# plot. Note how smooth continues past the points visible on this plot.
p + coord_cartesian(xlim = c(325, 500))
# By default, the same expansion factor is applied as when setting scale
# limits. You can set the limits precisely by setting expand = FALSE
p + coord_cartesian(xlim = c(325, 500), expand = FALSE)
# Simiarly, we can use expand = FALSE to turn off expansion with the
# default limits
p + coord_cartesian(expand = FALSE)
# You can see the same thing with this 2d histogram
d <- ggplot(diamonds, aes(carat, price)) +
stat_bin2d(bins = 25, colour = "white")
d
# When zooming the scale, the we get 25 new bins that are the same
# size on the plot, but represent smaller regions of the data space
d + scale_x_continuous(limits = c(0, 1))
#> Warning: Removed 17502 rows containing non-finite values (stat_bin2d).
# When zooming the coordinate system, we see a subset of original 50 bins,
# displayed bigger
d + coord_cartesian(xlim = c(0, 1))
