STUIXY - Inversely maps a single point on the streamline from user to data coordinate space.

       To use STUIXY, load the NCAR Graphics libraries ncarg, ncarg_gks, and ncarg_c, preferably in that order.

       Copyright (C) 1987-2009
       University Corporation for Atmospheric Research
       The use of this Software is governed by a License Agreement.

UNIX                                               April 1993                                     STUIXY(3NCARG)

       XUS         (REAL, input): The X coordinate of a point in the user coordinate system.

       YUS         (REAL, input): The Y coordinate of a point in the user coordinate system.

       XDA         (REAL, output): The X coordinate of the point in the data coordinate system.

       YDA         (REAL, output): The Y coordinate of the point in the data coordinate system.

       IST         (REAL,  output):  Status  of  the  mapping  operation:  0  indicates success, negative values
                   indicate that the mapping failed; positive values are reserved and should not be used by  the
                   implementor of a mapping routine.

       STUIXY - Inversely maps a single point on the streamline from user to data coordinate space.

       Online:  stgetc, stgeti, stgetr, stinit, stream, streamlines, streamlines_params, strset, stsetc, stseti,
       stsetr, stumsl, stumta, stumxy, ncarg_cbind.

       Hardcopy: NCAR Graphics Fundamentals, UNIX Version

       CALL STUIXY (XUS,YUS,XDA,YDA,IST)

       STUIXY is a user-modifiable routine provided to support custom mappings of the data coordinate space. The
       user does not invoke it directly.  Instead, whenever the parameter MAP specifies a mapping not handled by
       Streamlines internally (i.e., when MAP is set to a value other than 0, 1, or 2), Streamlines calls STUIXY
       once  for  each  incremental  step  in the creation of a streamline. The default version of STUIXY simply
       performs an identity mapping. In order to implement a custom mapping, you must pick a unique mapping code
       (a positive integer greater than 2), and then modify each of the  three  routines,  STUMXY,  STUIXY,  and
       STUMTA  to  recognize  and  respond consistently to the chosen code. In the standard distribution of NCAR
       Graphics, these three routines reside in a single file, ´stumxy.f´.  STUMXY maps a  point  from  data  to
       user  coordinate  space,  STUIXY  inversely  maps a point from user to data coordinate space, and STUMTA,
       which is likely to be the most difficult to implement, finds the tangent angle of  the  streamline  at  a
       point in NDC space.

       STUIXY  has  access to a common block called STMAP that contains a number of variables used to record the
       current transformation state. In order  to  accommodate  a  variety  of  mapping  implementations,  STMAP
       provides  more  information than normally required. Consider the values stored in STMAP as strictly read-
       only. One essential member of this common block is IMAP, which contains the value currently  assigned  to
       the MAP parameter.

       In STUIXY, the implementor of a custom mapping needs to add code of the form:

            IF (IMAP .EQ. <chosen_map_code>) THEN

                ...map XUS to XDA

                ...map YUS to YDA

                ...if error, set IST to a negative value

                RETURN

            ENDIF

       Note  that  after  the  return  from STUIXY, Streamlines checks to ensure that XDA and YDA are within the
       current boundaries of the data coordinate system.

       When implementing a custom mapping, you may wish to look at the coding of the  pre-defined  mappings  (0,
       identity  mapping;  1,  Ezmap  projections;  and 2, polar coordinate mapping) in the file ´stmpxy.f´. For
       these mappings, the subroutine STIMXY is the equivalent of STUIXY; it has an identical interface and  may
       serve as a model for your implementation.