Returns an object of class list containing the following components:
x1, x2minimum and maximum x value.
y1, y2minimum and maximum y value.
z1, z2minimum and maximum z value.
x1.chk, x2.chkif true, a default value is used for the minimum and maximum x value.
y1.chk, y2.chkif true, a default value is used for the minimum and maximum y value.
z1.chk, z2.chkif true, a default value is used for the minimum and maximum z value.
xvector of x limits (x1,x2), default is (NA,NA).
yvector of y limits (y1,y2), default is (NA,NA).
zvector of z limits (z1,z2), default is (NA,NA).