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+C
+C                                                                               
+C +-----------------------------------------------------------------+           
+C |                                                                 |           
+C |                Copyright (C) 1986 by UCAR                       |           
+C |        University Corporation for Atmospheric Research          |           
+C |                    All Rights Reserved                          |           
+C |                                                                 |           
+C |                 NCARGRAPHICS  Version 1.00                      |           
+C |                                                                 |           
+C +-----------------------------------------------------------------+           
+C                                                                               
+C                                                                               
+C ---------------------------------------------------------------------
+C
+      SUBROUTINE AGEXUS (SVAL,ZMIN,ZMAX,ZLOW,ZHGH,
+     +                               ZDRA,NVIZ,IIVZ,NEVZ,IIEZ,UMIN,UMAX)
+C
+      DIMENSION SVAL(2),ZDRA(1)
+C
+C The routine AGEXUS is used by AGSTUP to determine tentative values of
+C the user-window edge coordinates.  Its arguments are as follows:
+C
+C -- SVAL is the array of special values.
+C
+C -- ZMIN and ZMAX are user-supplied minimum and maximum values of the
+C    data x (or y) coordinates.
+C
+C -- ZLOW and ZHGH are, respectively, the smallest and largest data
+C    values to be considered in choosing the minimum and maximum, if
+C    those values, as given by the user, are null.
+C
+C -- ZDRA, NVIZ, IIVZ, NEVZ, and IIEZ specify the array of x (or y)
+C    data coordinates (see AGMAXI or AGMINI for complete description).
+C
+C -- UMIN and UMAX are returned with tentative minimum and maximum
+C    values for use at the appropriate user-window edges (left/right
+C    or bottom/top).
+C
+C The following common block contains AUTOGRAPH variables which are
+C not control parameters.  The only one used here is SMRL, which is a
+C (machine-dependent) small real which, when added to a number in the
+C range (1,10), will round it upward without seriously affecting the
+C leading significant digits.  The object of this is to get rid of
+C strings of nines.
+C
+      COMMON /AGORIP/ SMRL , ISLD , MWCL,MWCM,MWCE,MDLA,MWCD,MWDQ ,
+     +                INIF
+C
+C Assume initially that the user has provided actual values to be used.
+C
+      UMIN=ZMIN
+      UMAX=ZMAX
+C
+C If either of the values is null, replace it by a data-based value.
+C
+      IF (UMIN.EQ.SVAL(1).OR.UMIN.EQ.SVAL(2))
+     +                UMIN=AGMINI(SVAL(1),ZLOW,ZDRA,NVIZ,IIVZ,NEVZ,IIEZ)
+      IF (UMAX.EQ.SVAL(1).OR.UMAX.EQ.SVAL(2))
+     +                UMAX=AGMAXI(SVAL(1),ZHGH,ZDRA,NVIZ,IIVZ,NEVZ,IIEZ)
+C
+C Either or both values might still be null (if the user data was null).
+C
+      IF (UMIN.EQ.SVAL(1)) UMIN=UMAX
+      IF (UMAX.EQ.SVAL(1)) UMAX=UMIN
+C
+C Check the relative values of UMIN and UMAX for problems.
+C
+      IF (ABS(UMIN-UMAX).LT.50.*SMRL*(ABS(UMIN)+ABS(UMAX))) GO TO 102
+      IF (UMAX-UMIN) 101,102,103
+  101 IF (ZMIN.NE.SVAL(1).AND.ZMIN.NE.SVAL(2)) UMAX=UMIN
+      IF (ZMAX.NE.SVAL(1).AND.ZMAX.NE.SVAL(2)) UMIN=UMAX
+C
+  102 UMIN=UMIN-.5*ABS(UMIN)
+      UMAX=UMAX+.5*ABS(UMAX)
+      IF (UMIN.NE.UMAX) GO TO 103
+      UMIN=-1.
+      UMAX=+1.
+C
+C If the user wanted these values back-stored, do it.
+C
+  103 IF (ZMIN.EQ.SVAL(2)) ZMIN=UMIN
+      IF (ZMAX.EQ.SVAL(2)) ZMAX=UMAX
+C
+C Done.
+C
+      RETURN
+C
+      END