Working of ggplot scale continuous expand argument

Question

According to the scale_continuous documentation

A numeric vector of length two giving multiplicative and additive expansion constants. These constants ensure that the data is placed some distance away from the axes. The defaults are c(0.05, 0) for continuous variables, and c(0, 0.6) for discrete variables.

Since they are stated as "expansion constants", they are not actual units.

Can we convert them to some actual units to predict the actual output?

Answer 1

According to the document if you set limits manually, it would be more clear.

Refer below examples to know how it works:

The 1st argument gives expansion equal to multiplication by limit range.

ggplot(mpg, aes(displ, hwy)) +
    geom_point() +
    scale_x_continuous(limits = c(1, 7), expand = c(0.5, 0))
# Right most position will be 7 + (7-1) * 0.5 = 10

The 2nd argument gives the absolute expansion added to both end of the axis:

ggplot(mpg, aes(displ, hwy)) +
    geom_point() +
    scale_x_continuous(limits = c(1, 7), expand = c(0.5, 2))
# Right most position will be 7 + (7-1) * 0.5  + 2 = 12