ttools: generate multi-coordinate coord names consistently

Add a new method PlotUtil.getIndexSuffix which is used consistently
wherever multi-position coordinate sets are used.  This allows them
to be identified by name without guesswork.

topcat/src/main/uk/ac/starlink/topcat/plot2/PositionCoordPanel.java

@@ -6,6 +6,7 @@
 import uk.ac.starlink.table.DomainMapper;
 import uk.ac.starlink.table.ValueInfo;
 import uk.ac.starlink.ttools.plot2.DataGeom;
+import uk.ac.starlink.ttools.plot2.PlotUtil;
 import uk.ac.starlink.ttools.plot2.data.Coord;
 import uk.ac.starlink.ttools.plot2.data.StorageType;
 
@@ -69,7 +70,7 @@ public static Coord[] multiplyCoords( Coord[] coords, int ncopy ) {
      * @return   new coord like the input one
      */
     private static Coord relabel( final Coord baseCoord, int iPoint ) {
-        String iptxt = Integer.toString( iPoint + 1 );
+        String iptxt = PlotUtil.getIndexSuffix( iPoint );
         final ValueInfo[] infos = baseCoord.getUserInfos().clone();
         int nuc = infos.length;
         for ( int iuc = 0; iuc < nuc; iuc++ ) {

ttools/src/main/uk/ac/starlink/ttools/plot2/PlotUtil.java

@@ -93,6 +93,22 @@ public static boolean storeFullPrecision() {
         return true;
     }
 
+    /**
+     * Returns a suffix to append to one of a set of similar coordinate
+     * names for disambiguation.
+     * Where there are several sets of positional coordinates that would
+     * otherwise have the same names, use this method to come up with
+     * a consistent suffix.  It is only usual to invoke this method
+     * if there are in fact multiple positions, if there's only one just
+     * don't give it a suffix.
+     * 
+     * @param  ipos  zero-based position number
+     * @return  suffix; currently <code>1+ipos</code>
+     */
+    public static String getIndexSuffix( int ipos ) {
+        return Integer.toString( 1 + ipos );
+    }
+
     /**
      * Writes message through the logging system
      * about the time a named step has taken.

ttools/src/main/uk/ac/starlink/ttools/plot2/task/Plot2Task.java

@@ -752,7 +752,7 @@ private DataSpec createDataSpec( Environment env, String suffix,
         List<CoordValue> cvlist = new ArrayList<CoordValue>();
         for ( int ipos = 0; ipos < npos; ipos++ ) {
             Coord[] posCoords = geom.getPosCoords();
-            String posSuffix = npos > 1 ? Integer.toString( ipos + 1 ) : "";
+            String posSuffix = npos > 1 ? PlotUtil.getIndexSuffix( ipos ) : "";
             for ( int ic = 0; ic < posCoords.length; ic++ ) {
                 cvlist.add( getCoordValue( env, posCoords[ ic ],
                                            posSuffix + suffix ) );