Quote faceting variables

Just like aes(), vars() is a quoting function that takes inputs to be evaluated in the context of a dataset. These inputs can be:

• variable names

• complex expressions

In both cases, the results (the vectors that the variable represents or the results of the expressions) are used to form faceting groups.

vars(...)

Arguments

...	Variables or expressions automatically quoted. These are evaluated in the context of the data to form faceting groups. Can be named (the names are passed to a labeller).

See also

aes(), facet_wrap(), facet_grid()

Examples

p <- ggplot(mtcars, aes(wt, disp)) + geom_point()
p + facet_wrap(vars(vs, am))

# vars() makes it easy to pass variables from wrapper functions:
wrap_by <- function(...) {
  facet_wrap(vars(...), labeller = label_both)
}
p + wrap_by(vs)
p + wrap_by(vs, am)

# You can also supply expressions to vars(). In this case it's often a
# good idea to supply a name as well:
p + wrap_by(drat = cut_number(drat, 3))

# Let's create another function for cutting and wrapping a
# variable. This time it will take a named argument instead of dots,
# so we'll have to use the "enquote and unquote" pattern:
wrap_cut <- function(var, n = 3) {
  # Let's enquote the named argument `var` to make it auto-quoting:
  var <- enquo(var)

  # `quo_name()` will create a nice default name:
  nm <- quo_name(var)

  # Now let's unquote everything at the right place. Note that we also
  # unquote `n` just in case the data frame has a column named
  # `n`. The latter would have precedence over our local variable
  # because the data is always masking the environment.
  wrap_by(!!nm := cut_number(!!var, !!n))
}

# Thanks to tidy eval idioms we now have another useful wrapper:
p + wrap_cut(drat)