Thanks, I got what you want to say,
I have tried with your solution
Qt Code: 
  1. connect(m_plot->axisWidget(QwtPlot::xBottom), &QwtScaleWidget::scaleDivChanged, this, &OscilloscopeWidget::scaleChanged);
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Qt Code: 
  1. void OscilloscopeWidget::scaleChanged()
  2. {
  3.     qDebug()<<"Scale Changed";
  4. }
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But getting this error below, Where is the mistake ?
oscilloscopewidget.obj:-1: error: LNK2019: unresolved external symbol "public: static struct QMetaObject const QwtScaleWidget::staticMetaObject" (?staticMetaObject@QwtScaleWidget@@2UQMetaObject@@ B) referenced in function "public: static class QMetaObject::Connection __cdecl QObject::connect<void (__cdecl QwtScaleWidget::*)(void),void (__cdecl OscilloscopeWidget::*)(void)>(class QwtScaleWidget const *,void (__cdecl QwtScaleWidget::*)(void),class OscilloscopeWidget const *,void (__cdecl OscilloscopeWidget::*)(void),enum Qt::ConnectionType)" (??$connect@P8QwtScaleWidget@@EAAXXZP8Oscilloscope Widget@@EAAXXZ@QObject@@SA?AVConnection@QMetaObjec t@@PEBVQwtScaleWidget@@P83@EAAXXZPEBVOscilloscopeW idget@@P84@EAAXXZW4ConnectionType@Qt@@@Z)