STUMXY - Maps a single point on the streamline from data to user coordinate space.

       To use STUMXY, 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                                     STUMXY(3NCARG)

       XDA         (REAL, input): The X coordinate of a point on the streamline in data coordinate space.

       YDA         (REAL, input): The Y coordinate of a point on the streamline in data coordinate space.

       XUS         (REAL, output): The X coordinate of the point in user coordinate space.

       YUS         (REAL, output): The Y coordinate of the point in user coordinate space.

       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.

       STUMXY - Maps a single point on the streamline from data to user coordinate space.

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

       Hardcopy: NCAR Graphics Fundamentals, UNIX Version

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

       STUMXY 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 STUMXY
       as needed during the process of creating a streamline. The default version of STUMXY 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.

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

            IF (IMAP .EQ. <chosen_map_code>) THEN

                ...map XDA to XUS

                ...map YDA to YUS

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

                RETURN

            ENDIF

       Note  that  after  the  return  from STUMXY, Streamlines checks to ensure that XUS and YUS are within the
       current boundaries of the user 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 STMPXY is the equivalent of STUMXY; it has an identical interface and  may
       serve as a model for your implementation.