Hi Philminder, I tried your example under 6.00-rc5, and it fails at:

..\datetime\mainwindow.cpp: In constructor 'MainWindow::MainWindow(QWidget*)':
..\datetime\mainwindow.cpp:40: error: no matching function for call to 'QwtPlotCurve::setData(double*, double*, int)'
..\libraries\qwt/qwt_plot_seriesitem.h:146: note: candidates are: void QwtPlotSeriesItem<T>::setData(QwtSeriesData<T>*) [with T = QPointF]

It looks like setData arguments have changed...