Uwe,

I'd done it by your advice#3. And it works as expected exactly.
Thank you very much.

Qt Code: 
  1. TrendPlotLegend::TrendPlotLegend(QWidget* parent)
  2.     : QwtLegend(parent)
  3. {
  4.     contentsWidget()->layout()->setAlignment(Qt::AlignRight | Qt::AlignTop);
  5.  
  6.     QHBoxLayout* hbLayout = new QHBoxLayout(this);
  7.     hbLayout->setContentsMargins(10, 0, 5, 0);
  8.     hbLayout->setSpacing(0);
  9.  
  10.     m_footer = new QLabel(this);
  11.     hbLayout->addWidget(m_footer, 0, Qt::AlignTop);
  12.  
  13.     // the old layout is a QVBoxLayout/ replace it with QHBoxLayout
  14.     QLayout* oldLayout = layout();
  15.     for (int i = 0; i < oldLayout->count(); ++i) {
  16.         hbLayout->addItem(oldLayout->takeAt(i));
  17.     }
  18.     delete oldLayout;
  19.     setLayout(hbLayout);
  20. }
  21.  
  22. QSize TrendPlotLegend::sizeHint() const
  23. {
  24.     QSize size = QwtLegend::sizeHint();
  25.  
  26.     int width = size.width() + m_footer->sizeHint().width();
  27.     size.setWidth(qMax(parentWidget()->width(), width));
  28.     return size;
  29. }
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