In function
void QwtPlotZoomer::zoom( int offset )
{
if ( offset == 0 )
d_data->zoomRectIndex = 0;
else
{
int newIndex = d_data->zoomRectIndex + offset;
newIndex = qMax( 0, newIndex );
newIndex = qMin( int( d_data->zoomStack.count() ) - 1, newIndex );

d_data->zoomRectIndex = uint( newIndex );
}

rescale();

Q_EMIT zoomed( zoomRect() );
}

need to generate |zoomed| and to call |rescale| only if value of |d_data->zoomRectIndex| is changed
???