m_model.setValueRange(0, maxValue, 0, maxValue);
// perform second walk to compute polygon layout
Table t = (Table)m_vis.getGroup(m_group);
Iterator iter = t.tuplesReversed();
while ( iter.hasNext() ) {
VisualItem item = (VisualItem)iter.next();
if ( !item.isVisible() ) continue;
float height = 0;