There are three types of variables in GGobi displays:
x, y, z, which correspond to the labels on the toggle buttons
in GGobi. Most plots have a constrained set of possible options.
For example, in tours you can only set x variables, and you must
have at least three. Or in the rotation plot, you need exactly
one x, y, and z variable.
Currently, there is no checking done to ensure that you are
sending a sensible set of variables for the given display type.
Generally, any invalid choices will be silently ignored.