expand_btn.setToolTipText(
"Expand the Y-axis tree label by 1 level" );
expand_btn.setMnemonic( KeyEvent.VK_E );
// expand_btn.setPreferredSize( btn_dim );
expand_btn.addActionListener(
new ActionYaxisTreeExpand( this, y_tree ) );
super.add( expand_btn );
icon_URL = getURL( Const.IMG_PATH + "TreeCollapse24.gif" );
if ( icon_URL != null )
collapse_btn = new JButton( new ImageIcon( icon_URL ) );