Facetting fails with "undefined columns selected" error for non-standard variable names

[richierocks]

To reproduce:

library(ggplot2)

DF <- data.frame(
  x = 1:10,
  y = runif(10),
  `z z` = gl(5, 2),
  check.names = FALSE
)

ggplot(DF, aes(x, y)) +
  geom_point() +
  facet_wrap(~ `z z`)
## Error in `[.data.frame`(panels, missing_facets) : 
##   undefined columns selected

ggplot(DF, aes(x, y)) +
  geom_point() +
  facet_grid(`z z` ~ .)
## Error in `[.data.frame`(base, names(rows)) : undefined columns selected

[smouksassi]

you might want to have a look at:
http://stackoverflow.com/questions/12744282/how-to-deal-with-spaces-in-column-names
the back tick works in aesthetics but not in facets:
DF <- data.frame(
x = 1:10,
y y = runif(10),
z z = gl(5, 2),
check.names = FALSE
)
ggplot(DF, aes(x, y y)) +
geom_point()

[richierocks]

Thanks Samer.

It was the fact that backticks work in aesthetics that led me to believe that they should work in facets too.

For the record, using the standardised version of the column name doesn't work either.

ggplot(DF, aes(x, y)) +
  geom_point() +
  facet_wrap(~ z.z)

If no proper fix is possible (that is, making non-standard names correctly draw facets), then It would be nice to provide a more informative error message.

For example, at the start of facet_wrap, you can check for valid variable names. If you are happy to have a dependency on assertive, then this will do the trick:

library(assertive)
assert_all_are_valid_variable_names(attr(terms(facets), "term.labels"))

(The case for facet_grid is slightly harder because stats::terms is silly and you have to get the LHS separately.)

A note in the ?facet_wrap/?facet_grid help pages might be nice too.