Labeller functions are in charge of formatting the strip labels of
facet grids and wraps. Most of them accept a
argument to control whether multiple factors (defined in formulae
~first + second) should be displayed on a single
line separated with commas, or each on their own line.
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Data frame of labels. Usually contains only one element, but facetting over multiple factors entails multiple label variables.
Whether to display the labels of multiple factors on separate lines.
String separating variables and values.
Maximum number of characters before wrapping the strip.
label_value() only displays the value of a factor while
label_both() displays both the variable name and the factor
label_context() is context-dependent and uses
label_value() for single factor facetting and
label_both() when multiple factors are
for line wrapping.
label_parsed() interprets the labels as plotmath
label_bquote() offers a more flexible
way of constructing plotmath expressions. See examples and
bquote() for details on the syntax of the
Writing New Labeller Functions
Note that an easy way to write a labeller function is to
transform a function operating on character vectors with
A labeller function accepts a data frame of labels (character
vectors) containing one column for each factor. Multiple factors
occur with formula of the type
~first + second.
The return value must be a rectangular list where each 'row' characterises a single facet. The list elements can be either character vectors or lists of plotmath expressions. When multiple elements are returned, they get displayed on their own new lines (i.e., each facet gets a multi-line strip of labels).
To illustrate, let's say your labeller returns a list of two character vectors of length 3. This is a rectangular list because all elements have the same length. The first facet will get the first elements of each vector and display each of them on their own line. Then the second facet gets the second elements of each vector, and so on.
If it's useful to your labeller, you can retrieve the
attribute of the incoming data frame of labels. The value of this
attribute reflects the kind of strips your labeller is dealing
"cols" for columns and
"rows" for rows. Note
facet_wrap() has columns by default and rows
when the strips are switched with the
switch option. The
facet attribute also provides metadata on the labels. It
takes the values
For compatibility with
labeller(), each labeller
function must have the
labeller S3 class.
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mtcars$cyl2 <- factor(mtcars$cyl, labels = c("alpha", "beta", "gamma")) p <- ggplot(mtcars, aes(wt, mpg)) + geom_point() # Displaying only the values p + facet_grid(. ~ cyl) p + facet_grid(. ~ cyl, labeller = label_value) # Displaying both the values and the variables p + facet_grid(. ~ cyl, labeller = label_both) # Displaying only the values or both the values and variables # depending on whether multiple factors are facetted over p + facet_grid(am ~ vs+cyl, labeller = label_context) # Interpreting the labels as plotmath expressions p + facet_grid(. ~ cyl2) p + facet_grid(. ~ cyl2, labeller = label_parsed) p + facet_wrap(~vs + cyl2, labeller = label_parsed)