@Override
 public String getVertexToolTip(String object) {
   Tree tree = vertexMap.get(object);
   if (tree != null) {
     StringBuffer result = new StringBuffer();
     if (tree.isLeaf()) {
       String labelString = tree.getLabel();
       if (labelString != null) {
         result.append("<html><b>Class:</b>&nbsp;" + labelString + "<br>");
         result.append("<b>Size:</b>&nbsp;" + tree.getFrequencySum() + "<br>");
         result.append(
             "<b>Class frequencies:</b>&nbsp;"
                 + SwingTools.transformToolTipText(tree.getCounterMap().toString())
                 + "</html>");
       }
     } else {
       result.append(
           "<html><b>Subtree Size:</b>&nbsp;" + tree.getSubtreeFrequencySum() + "</html>");
     }
     return result.toString();
   } else {
     return null;
   }
 }