Add column groups of an object of ztable
addcgroup(z, cgroup, n.cgroup, color = "black", bg = "white", top = FALSE)An object of ztable
A character vector or matrix indicating names of column group. Default value is NULL
A integer vector or matrix indicating the numbers of columns included in each cgroup Default value is NULL
A character vector indicating the font color of each cells.
A character vector indicating the background color of each cells.
Logical. Whether or not cgroup be placed at top.