Tour Manipulation Constraints

The variable to be manipulated into or out of the plot is indicated by
an inner circle in the variable circle. The control follows pointer
movement on the plot window.

Variable to be manipulated is indicated in the variable circle by a
small circle. 

Oblique: No constraints on manipulating a variable

Vert: Variable manipulation is constrained along a vertical
latitude line.

Horiz: Variable manipulation is constrained along a horizontal
latitude line. 

Radial: Variable manipulation is constrained to direction of variable
contribution in the plane. If the manipulated variable doesn't have
any current contribution the first mouse movement direction is used.

Angle: Variable manipulation is constrained to rotation in the current
plane. The angle of rotation is determined by movement around the
center of the plot window, so mouse motion near the center causes
dramatic changes!

USAGE WARNING: There is one case where the controls for the manual tour
is unresolved:
  When PC Basis is on and the Manip variable is not one of the active
  variables. The active variables are sphered but the Manip variable is
  not!