Drawing a QwtPlot object element on top of other elements (QWT library)

Good day! Help me please. It is necessary to draw the QwtPlotMarker element on top of the QwtCurvePlot to improve rendering performance (to bypass the replot() method, which rebuilds the entire contents of the QwtPlot).

I am attaching a link to the example itemeditor

<https://github.com/opencor/qwt/tree/master/examples/itemeditor>

I'm trying to create my own QwtWidgetOverlay class similar to the example from QWT itemeditor, but I don't understand where to start

Start with copying the Editor class and replace QwtPlotShapeItem by QwtPlotMarker.

HTH,
Uwe