cgroup
A vector or a matrix of character strings defining major column header. The default
is to have none. These elements are also known as column spanners. If you want a column not
to have a spanner then put that column as "". If you pass cgroup and n.crgroup as
matrices you can have column spanners for several rows. See cgroup section below for details.
n.cgroup
An integer vector or matrix containing the number of columns for which each element in
cgroup is a heading. For example, specify cgroup=c("Major_1","Major_2"),
n.cgroup=c(3,3) if "Major_1" is to span columns 1-3 and
"Major_2" is to span columns 4-6.
rowlabel does not count in the column numbers. You can omit n.cgroup
if all groups have the same number of columns. If the n.cgroup is one less than
the number of columns in the matrix/data.frame then it automatically adds those.