В общем, встроенная справка довольно хороша.Вы правы, хотя на странице справки для boxplot упоминается na.action без указания параметров.
В этом случае ?na.action и - после этого - ?na.omit объяснениевозможности (они носят общий характер и также относятся к вещам, отличным от boxplot.)
Handle Missing Values in Objects
Description:
These generic functions are useful for dealing with ‘NA’s in e.g.,
data frames. ‘na.fail’ returns the object if it does not contain
any missing values, and signals an error otherwise. ‘na.omit’
returns the object with incomplete cases removed. ‘na.pass’
returns the object unchanged.
Usage:
na.fail(object, ...)
na.omit(object, ...)
na.exclude(object, ...)
na.pass(object, ...)
Arguments:
object: an R object, typically a data frame
...: further arguments special methods could require.
Details:
At present these will handle vectors, matrices and data frames
comprising vectors and matrices (only).
If ‘na.omit’ removes cases, the row numbers of the cases form the
‘"na.action"’ attribute of the result, of class ‘"omit"’.
‘na.exclude’ differs from ‘na.omit’ only in the class of the
‘"na.action"’ attribute of the result, which is ‘"exclude"’. This
gives different behaviour in functions making use of ‘naresid’ and
‘napredict’: when ‘na.exclude’ is used the residuals and
predictions are padded to the correct length by inserting ‘NA’s
for cases omitted by ‘na.exclude’.