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   This function creates a GUI for grouping variables
     used by the Plotting Tool.
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   Form:
   x = DisplayFilter( action, modifier  )
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   ------
   Inputs
   ------
   action          (1,:)   Action to be taken by the gui (used mostly
                           for callbacks)
   modifier        (1,:)   Modifier to the action

   -------
   Outputs
   -------
   x

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