one or more data vectors, which may be given in
name=value form.
VALUE:
The input vectors
are converted to a data frame with components named
data and
which.
data
a vector made up by concatenating the data values from all the input series.
which
a factor which tells which input argument each value comes from.
The levels of this factor are the expressions given in
...,
unless the arguments are given by name, in which case the given
name is used.