Hide or exclude groups

Use this panel to manipulate the brushed groups, where each group
is defined as a set of cases having the same glyph and color.  As
soon as you open this panel, a new variable, labelled "group," is
added to the variable selection panel.

Reset brushed groups

  Click here to redefine the groups after brushing some points on
  the screen.

The groups panel contains one row for each group, and there are
four columns. 

The group name is assigned to be the group number,
but you can edit that field.  The names you assign will be retained
if you press "Reset brushed groups," but it isn't presently saved
between subsequent xgobi runs.  (We'll have to add a new file
format to hang on to them.)

The symbol is the glyph and color of the group -- and it's also a
button.  Clicking on it will reassign the glyph and color for all
members of a group to the glyph and color currently selected in the
brushing panel.

Hidden:  Group members are not drawn.
Excluded:  Group members are excluded from all calculations (scaling,
  sphering, principal coordinates, projection pursuit) -- and they are
  <also> not drawn.

We know this is a bit ambiguous -- the interface might be more natural
if those categories were mutually exclusive, but that's not how the code
works.  There can be no cases that are excluded and drawn, because all
the routines that loop over rows pay no attention to the excluded cases,
which speeds up a lot of computations.  We hope this design will make
sense to you if you play with it a bit.