The Distance handle is used to allow the interactive choice of a
distance along an arbitrary vector.  This is visualized with several
blue circles that can be dragged along the vector.

Note that as the px, py, pz, vx, vy, and vz inputs are all read only,
they can map to the same channels as a Vector handle, thus allowing
one to have control over both the vector and the distance.

Parameters:

    dist        - The distance from the origin along the vector which
                  the handle has been moved.
    input       - A group field.  The centroid of this group is used
                  as the default origin.
    onoff       - Whether the handle is visible, 1 means visible.
    px, py, pz  - The origin of the drag vector
    vx, vy, vz  - The normalized direction of the drag vector.

See Also: Vector
