FILTER

     FILTER is a logical variable or real number which controls the
     filtering of data in order to eliminate plotting of overlapping
     data.

     If FILTER is YES, the data will be filtered.

     If FILTER is NO, 0, or blank, all data will be plotted.

     If FILTER is set to a real number, the default filter will be scaled 
     by that number.  FILTER = 1 corresponds to FILTER = YES.
     0 < FILTER < 1 allows some data overlap.  FILTER > 1 causes data to 
     be more widely spaced.