size_button.signal_button_release_event().connect (mem_fun (*this, &VisualTimeAxis::size_click)) ;
visual_button.signal_clicked().connect (mem_fun (*this, &VisualTimeAxis::visual_click)) ;
hide_button.signal_clicked().connect (mem_fun (*this, &VisualTimeAxis::hide_click)) ;
size_button.signal_button_release_event().connect (mem_fun (*this, &VisualTimeAxis::size_click)) ;
visual_button.signal_clicked().connect (mem_fun (*this, &VisualTimeAxis::visual_click)) ;
hide_button.signal_clicked().connect (mem_fun (*this, &VisualTimeAxis::hide_click)) ;