XYGraph__DisplayCoordinates (ul_pbar_graphics)

 *  Synopsis:
 *    #include <ul_pbar_graphics:xygraph.h>
 *
 *    TXYGraph__Result XYGraph__DisplayCoordinates( void )
 *
 *  Function intent:
 *    XYGraph__DisplayCoordinates() processes the cursor tracking of X-Y
 *    coordinates in the plot window.  XYGraph__DisplayCoordinates() must be
 *    called at a regular rate (typically within pgm_per()) to be responsive to
 *    the user's actions.
 *  Function parameters:
 *    none
 *  Function return value:
 *    CBS_OK		success
 *    CBS_NOSETUP	XYGraph__DisplayGraph() has not been called or...
 *              	XYGraph__EnableCoordinates() not been called
 *    else 		wn_xxx() function error occured