get_gg_label: Checking Min, Max, Labels and Label Positions
In plothelper: New Plots Based on 'ggplot2' and Functions to Create Regular Shapes

Given a numeric vector or a ggplot object, the function will check the range, labels and label positions (the same as major grid lines) that will used on the axis. The result is a length 5 list for min limit, max limit, labels, major grid-line positions, all (major and minor) grid-line positions.

get_gg_label(
  a = NULL,
  b = NULL,
  v = NULL,
  gg = NULL,
  mult = 0.05,
  add = 0,
  axis = "y"
)

`a`	extreme values of a numeric vector. Note: only one of `a`, `v`, `gg` can be non-NULL. It can also be a gg object.
`b`	another extreme value if `a` is not NULL.
`v`	a numeric vector.
`gg`	a gg object created by ggplot function. Which value will be checked depends on `axis`.
`mult`	default is 0.05 and should be of length 1 or 2. It mimics the `mult` argument of `ggplot2::expansion`. It is only used when a is numeric or v is non-NULL.
`add`	default is 0. It mimics the `add` argument of `ggplot2::expansion`.
`axis`	if `gg` is used or `a` is a ggplot object, which axis will be checked? It can be "x" or "y" (default).

get_gg_label(a=1, b=1000)
# The following three have the same results.
get_gg_label(a=1, b=1000, mult=0)
get_gg_label(v=c(1, 500, 1000), mult=0)
p=ggplot()+geom_point(aes(1: 3, c(1, 500, 1000)))+
  scale_y_continuous(expand=expansion(mult=0))
get_gg_label(gg=p)