"VI", "VS", or "both"
"VI" give graphs with varying variance component of intercept and
with a fixed variance component for slope specified in vs argument
"VS" give graphs with varying variance component for slope and with a
fixed variance component of intercept specified in vi argument
"both" 3-D plot see also fun3D argument
vi
VI for which plots the output. Necessary for "VS" type of graph
vs
VS for which plots the output. Necessary for "VI" type of graph
fun3D
plot function used to plot the 3D graph.
"wireframe" uses lattice,
"persp" uses graphics,
and "open3d" uses rgl.