Thanks for your response.

i just created a pointer to QwtPlotMagnifier,
QwtPlotMagnifier * m_ptrzoomIn = new QwtPlotMagnifier(plot()->canvas());
m_ptrzoomIn->setMouseButton(Qt::RightButton,Qt::ControlModifie r);

i am able to zoomIn and zoomout but if i release the right button it is coming to the original position.