Correlation Tour:

This initiates a correlation (alias regression or canonical) tour with
the first variable as the "dependent" vertical variable and the second
two variables as "independent" horizontal variables. See Buja, Hurley,
Asimov and McDonald "Elements of a Viewing Pipeline" in Cleveland and
McGill (1989?) "Dynamic Graphics for Statistics".

Variables are added or removed with either a left or middle button
click. Left for X variable, Middle for Y variable. When removed the
variable will be faded out. To change from an X variable to a Y
variable one needs to click on the variable circle with the middle
button, and when the X component is faded out the Y component is
introduced. Vice versa for changing a Y to an X. 

This is the basic no-frills model!
