Backtrack

The tour that is implemented is an interpolation type tour generated by
essentially two steps:
  1. Generate 2-dimensional planes in p-space, either randomly or
some other desired manner; we call these the basis planes,
  2. Interpolate between the basis planes.

As tour runs, a history file of the base planes passed is saved.  When
Backtrack is selected the tour runs backward through this path.
Simultaneously, the number in the box to the right of the Backtrack
button changes to display the number of the plane in the history file
that the tour is approaching.

When the tour has backtracked to the first plane in the history file,
it turns around and moves forward through the history file.  This is
indicated by the button to the right of Backtrack which instantaneously
changes from B (Backward) to F (Forward). Similarly, when the tour has
reached the end of the history file, it reverses direction again and
begins to move backward through the recorded planes, and the button to
the right of Backtrack correspondingly changes to B.  When Backtrack is
deactivated the tour begins again from the current projection
overwriting the history file from this position onward.

During Backtrack it is not possible to add or remove variables, since
the purpose is to retrace old steps.

This facility is also useful for interpolating between views found
by the projection pursuit guided tour, which have been recorded by
the Record Bitmap facility, and read back in via the I/O menu ReadHist
option.