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-ui/src/main/java/org/lucares/pdbui/PlotSettingsTransformer.java

@@ -80,6 +80,8 @@ class PlotSettingsTransformer {
       }
     }
 
+    aggregateHandlerCollection.updateAxisForHandlers();
+    
     return aggregateHandlerCollection;
   }
 }