SUBROUTINE PLOT6(Y,X,N,YMIN,YMAX,XMIN,XMAX) 
C
C     PURPOSE--THIS SUBROUTINE YIELDS A ONE-PAGE PRINTER PLOT
C              OF Y(I) VERSUS X(I): 
C              1) WITH THE VERTICAL (Y) AXIS MIN AND MAX
C                 AND THE HORIZONTAL (X) AXIS MIN AND MAX
C                 VALUES SPECIFIED BY THE DATA ANALYST.
C
C              THE USE OF THE YMIN, YMAX, XMIN, AND XMAX
C              SPECIFICATIONS ALLOWS THE DATA ANALYST
C              TO CONTROL FULLY THE PLOT AXIS LIMITS,
C              SO AS, FOR EXAMPLE, TO ZERO-IN ON AN
C              INTERESTING SUB-REGION OF A PREVIOUS PLOT.
C
C     INPUT ARGUMENTS--Y      = THE SINGLE PRECISION VECTOR OF
C                               (UNSORTED OR SORTED) OBSERVATIONS
C                               TO BE PLOTTED VERTICALLY.
C                    --X      = THE SINGLE PRECISION VECTOR OF
C                               (UNSORTED OR SORTED) OBSERVATIONS
C                               TO BE PLOTTED HORIZONTALLY. 
C                    --N      = THE INTEGER NUMBER OF OBSERVATIONS
C                               IN THE VECTOR Y.
C                    --YMIN   = THE SINGLE PRECISION VALUE OF
C                               DESIRED MINIMUM FOR THE VERTICAL AXIS.
C                    --YMAX   = THE SINGLE PRECISION VALUE OF
C                               DESIRED MAXIMUM FOR THE VERTICAL AXIS.
C                    --XMIN   = THE SINGLE PRECISION VALUE OF
C                               DESIRED MINIMUM FOR THE HORIZONTAL AXIS.
C                    --XMAX   = THE SINGLE PRECISION VALUE OF
C                               DESIRED MAXIMUM FOR THE HORIZONTAL AXIS.
C     OUTPUT--A ONE-PAGE PRINTER PLOT OF Y(I) VERSUS X(I),
C             WITH SPECIFIED AXIS LIMITS.
C     PRINTING--YES.
C     RESTRICTIONS--THERE IS NO RESTRICTION ON THE MAXIMUM VALUE
C                   OF N FOR THIS SUBROUTINE.
C     OTHER DATAPAC   SUBROUTINES NEEDED--NONE.
C     FORTRAN LIBRARY SUBROUTINES NEEDED--NONE.
C     MODE OF INTERNAL OPERATIONS--SINGLE PRECISION.
C     LANGUAGE--ANSI FORTRAN. 
C     COMMENT--VALUES IN THE VERTICAL AXIS VECTOR (Y)
C              WHICH ARE SMALLER THAN YMIN OR LARGER THAN YMAX,
C              OR VALUES IN THE HORIZONTAL AXIS VECTOR (X)
C              WHICH ARE SMALLER THAN XMIN OR LARGER THAN XMAX
C              WILL NOT BE PLOTTED.
C            --VALUES IN THE VERTICAL AXIS VECTOR (Y)
C              OR THE HORIZONTAL AXIS VECTOR (X) WHICH ARE
C              EQUAL TO OR IN EXCESS OF 10.0**10 WILL NOT BE
C              PLOTTED.
C              THIS CONVENTION GREATLY SIMPLIFIES THE PROBLEM
C              OF PLOTTING WHEN SOME ELEMENTS IN THE VECTOR Y
C              (OR X) ARE 'MISSING DATA', OR WHEN WE PURPOSELY
C              WANT TO IGNORE CERTAIN ELEMENTS IN THE VECTOR Y
C              (OR X) FOR PLOTTING PURPOSES (THAT IS, WE DO NOT
C              WANT CERTAIN ELEMENTS IN Y (OR X) TO BE PLOTTED).
C              TO CAUSE SPECIFIC ELEMENTS IN Y (OR X) TO BE 
C              IGNORED, WE REPLACE THE ELEMENTS BEFOREHAND
C              (BY, FOR EXAMPLE, USE OF THE   REPLAC   SUBROUTINE)
C              BY SOME LARGE VALUE (LIKE, SAY, 10.0**10) AND
C              THEY WILL SUBSEQUENTLY BE IGNORED IN THE PLOT
C              SUBROUTINE.
C     REFERENCES--NONE.
C     WRITTEN BY--JAMES J. FILLIBEN
C                 STATISTICAL ENGINEERING LABORATORY (205.03)
C                 NATIONAL BUREAU OF STANDARDS
C                 WASHINGTON, D. C. 20234
C                 PHONE--301-921-2315
C     ORIGINAL VERSION--JUNE      1972. 
C     UPDATED         --OCTOBER   1975. 
C     UPDATED         --NOVEMBER  1975. 
C     UPDATED         --FEBRUARY  1976. 
C     UPDATED         --FEBRUARY  1977. 
C     UPDATED         --JUNE      1977. 
C
C---------------------------------------------------------------------
C
      CHARACTER*4 IGRAPH
      CHARACTER*4 SBNAM1,SBNAM2
      CHARACTER*4 ALPH11,ALPH12,ALPH21,ALPH22,ALPH31,ALPH32 
      CHARACTER*4 BLANK,HYPHEN,ALPHAI,ALPHAX
      CHARACTER*4 ALPHAM,ALPHAA,ALPHAD,ALPHAN,EQUAL
C
      DIMENSION Y(1)
      DIMENSION X(1)
      DIMENSION YLABLE(11)
      COMMON /BLOCK1/ IGRAPH(55,130)
C
      DATA SBNAM1,SBNAM2/'PLOT','6   '/ 
      DATA ALPH11,ALPH12/'FIRS','T   '/ 
      DATA ALPH21,ALPH22/'SECO','ND  '/ 
      DATA ALPH31,ALPH32/'THIR','D   '/ 
      DATA BLANK,HYPHEN,ALPHAI,ALPHAX/' ','-','I','X'/
      DATA ALPHAM,ALPHAA,ALPHAD,ALPHAN,EQUAL/'M','A','D','N','='/
C
      IPR=6
      CUTOFF=(10.0**10)-1000.0
C
C     CHECK THE INPUT ARGUMENTS FOR ERRORS
C
      WRITE(IPR,998)
      IF(N.LT.1)GOTO52
      GOTO54
   52 WRITE(IPR,5)
      WRITE(IPR,10) 
      WRITE(IPR,15)ALPH31,ALPH32,SBNAM1,SBNAM2
      WRITE(IPR,20)N
      WRITE(IPR,5)
      RETURN
   54 CONTINUE
      IF(N.EQ.1)GOTO56
      GOTO58
   56 WRITE(IPR,5)
      WRITE(IPR,10) 
      WRITE(IPR,15)ALPH31,ALPH32,SBNAM1,SBNAM2
      WRITE(IPR,22)N
      WRITE(IPR,5)
      RETURN
   58 CONTINUE
C
      HOLD=Y(1)
      DO60I=2,N
      IF(Y(I).NE.HOLD)GOTO62
   60 CONTINUE
      WRITE(IPR,5)
      WRITE(IPR,10) 
      WRITE(IPR,15)ALPH11,ALPH12,SBNAM1,SBNAM2
      WRITE(IPR,30)HOLD
      WRITE(IPR,5)
      RETURN
   62 CONTINUE
      HOLD=X(1)
      DO64I=2,N
      IF(X(I).NE.HOLD)GOTO66
   64 CONTINUE
      WRITE(IPR,5)
      WRITE(IPR,10) 
      WRITE(IPR,15)ALPH21,ALPH22,SBNAM1,SBNAM2
      WRITE(IPR,30)HOLD
      WRITE(IPR,5)
      RETURN
   66 CONTINUE
C
      DO76I=1,N
      IF(Y(I).LT.CUTOFF)GOTO78
   76 CONTINUE
      WRITE(IPR,5)
      WRITE(IPR,10) 
      WRITE(IPR,15)ALPH11,ALPH12,SBNAM1,SBNAM2
      WRITE(IPR,32) 
      WRITE(IPR,33)CUTOFF
      WRITE(IPR,5)
      RETURN
   78 CONTINUE
      DO80I=1,N
      IF(X(I).LT.CUTOFF)GOTO82
   80 CONTINUE
      WRITE(IPR,5)
      WRITE(IPR,10) 
      WRITE(IPR,15)ALPH21,ALPH22,SBNAM1,SBNAM2
      WRITE(IPR,32) 
      WRITE(IPR,33)CUTOFF
      WRITE(IPR,5)
      RETURN
   82 CONTINUE
C
      N2=0
      DO96I=1,N
      IF(Y(I).LT.CUTOFF.AND.X(I).LT.CUTOFF)GOTO98 
      GOTO96
   98 N2=N2+1
      IF(N2.GE.2)GOTO99
   96 CONTINUE
      WRITE(IPR,5)
      WRITE(IPR,10) 
      WRITE(IPR,18)ALPH11,ALPH12,ALPH21,ALPH22
      WRITE(IPR,19)SBNAM1,SBNAM2
      WRITE(IPR,40) 
      WRITE(IPR,41)N2
      WRITE(IPR,5)
      RETURN
   99 CONTINUE
C
    5 FORMAT(1H ,'**************************************************',
     1'********************') 
   10 FORMAT(1H ,'                   FATAL ERROR                    ')
   15 FORMAT(1H ,'THE ',A4,A4,' INPUT ARGUMENT TO THE ',A4,A4,
     1' SUBROUTINE')
   18 FORMAT(1H ,'THE ',A4,A4,', AND ',A4,A4)
   19 FORMAT(1H ,'INPUT ARGUMENTS TO THE ',A4,A4,' SUBROUTINE')
   20 FORMAT(1H ,'IS NON-NEGATIVE (WITH VALUE = ',I8,1H))
   22 FORMAT(1H ,'HAS THE VALUE 1')
   30 FORMAT(1H ,'HAS ALL ELEMENTS = ',E15.8)
   32 FORMAT(1H ,'HAS ALL ELEMENTS IN EXCESS OF THE CUTOFF')
   33 FORMAT(1H ,'VALUE OF ',E15.8)
   40 FORMAT(1H ,'ARE SUCH THAT TOO MANY POINTS HAVE BEEN', 
     1' EXCLUDED FROM THE PLOT.')
   41 FORMAT(1H ,'ONLY ',I3,' POINTS ARE LEFT TO BE PLOTTED.')
C
C-----START POINT-----------------------------------------------------
C
C     DETERMINE THE VALUES TO BE LISTED ON THE LEFT VERTICAL AXIS
C
      DO400I=1,9
      AIM1=I-1
      YLABLE(I)=YMAX-(AIM1/8.0)*(YMAX-YMIN)
  400 CONTINUE
C
C     DETERMINE THE VALUES TO BE LISTED ON THE BOTTOM HORIZONTAL AXIS 
C     DETERMINE XMID, X25 (=THE 25% POINT), AND
C     X75 (=THE 75% POINT)
C
      XMID=(XMIN+XMAX)/2.0
      X25=0.75*XMIN+0.25*XMAX 
      X75=0.25*XMIN+0.75*XMAX 
C
C     BLANK OUT THE GRAPH
C
      DO1100I=1,45
      DO1200J=1,109 
      IGRAPH(I,J)=BLANK
 1200 CONTINUE
 1100 CONTINUE
C
C     PRODUCE THE VERTICAL AXES
C
      DO1300I=3,43
      IGRAPH(I,5)=ALPHAI
      IGRAPH(I,109)=ALPHAI
 1300 CONTINUE
      DO1400I=3,43,5
      IGRAPH(I,5)=HYPHEN
      IGRAPH(I,109)=HYPHEN
 1400 CONTINUE
      IGRAPH(3,1)=EQUAL
      IGRAPH(3,2)=ALPHAM
      IGRAPH(3,3)=ALPHAA
      IGRAPH(3,4)=ALPHAX
      IGRAPH(23,1)=EQUAL
      IGRAPH(23,2)=ALPHAM
      IGRAPH(23,3)=ALPHAI
      IGRAPH(23,4)=ALPHAD
      IGRAPH(43,1)=EQUAL
      IGRAPH(43,2)=ALPHAM
      IGRAPH(43,3)=ALPHAI
      IGRAPH(43,4)=ALPHAN
C
C     PRODUCE THE HORIZONTAL AXES
C
      DO1500J=7,107 
      IGRAPH(1,J)=HYPHEN
      IGRAPH(45,J)=HYPHEN
 1500 CONTINUE
      DO1600J=7,107,25
      IGRAPH(1,J)=ALPHAI
      IGRAPH(45,J)=ALPHAI
 1600 CONTINUE
      DO1700J=20,107,25
      IGRAPH(1,J)=ALPHAI
      IGRAPH(45,J)=ALPHAI
 1700 CONTINUE
C
C     DETERMINE THE (X,Y) PLOT POSITIONS
C
      RATIOY=40.0/(YMAX-YMIN) 
      RATIOX=100.0/(XMAX-XMIN)
      DO1800I=1,N
      IF(Y(I).GE.CUTOFF)GOTO1800
      IF(X(I).GE.CUTOFF)GOTO1800
      IF(Y(I).LT.YMIN.OR.Y(I).GT.YMAX)GOTO1800
      IF(X(I).LT.XMIN.OR.X(I).GT.XMAX)GOTO1800
      MX=RATIOX*(X(I)-XMIN)+0.5
      MX=MX+7
      MY=RATIOY*(Y(I)-YMIN)+0.5
      MY=43-MY
      IGRAPH(MY,MX)=ALPHAX
 1800 CONTINUE
C
C     WRITE OUT THE GRAPH
C
      DO2100I=1,45
      IP2=I+2
      IFLAG=IP2-(IP2/5)*5
      K=IP2/5
      IF(IFLAG.NE.0)WRITE(IPR,2105)(IGRAPH(I,J),J=1,109)
      IF(IFLAG.EQ.0)WRITE(IPR,2106)YLABLE(K),(IGRAPH(I,J),J=1,109)
 2100 CONTINUE
      WRITE(IPR,2107)XMIN,X25,XMID,X75,XMAX
C
 2105 FORMAT(1H ,20X,109A1)
 2106 FORMAT(1H ,F20.7,109A1) 
 2107 FORMAT(1H ,14X,F20.7,5X,F20.7,5X,F20.7,5X,F20.7,1X,F20.7)
  998 FORMAT(1H1)
C
      RETURN
      END