make the code that determines which axis to use explicit

In the previous changeset the code that determined
which axis the plots used was implemented as a
side effect of getting the Gnuplot definition of
an axis.
Changed that to an explit update call with simpler
logic.

pdb-utils/src/main/java/org/lucares/utils/CollectionUtils.java

@@ -2,6 +2,8 @@ package org.lucares.utils;
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
+import java.util.Comparator;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -37,6 +39,12 @@ public class CollectionUtils {
   }
   
   
+  public static <T> List<T> copySort(Collection<? extends T> collection, Comparator<T> comparator){
+    final List<T> result = new ArrayList<T>(collection);
+    Collections.sort(result, comparator);
+    return result;
+  }
+  
 	public static <T, R extends T> void mapInPlace(final List<T> list, final Function<T, R> mapper) {
 		for (int i = 0; i < list.size(); i++) {
 			final T value = list.get(i);