Use a picker with a RectRubberBand and one of the machines made for rectangles ( f.e. QwtPickerDragRectMachine ). Overload QwtPicker::drawRubberband() where you do a QPainter::drawRect() using the y coordinates from of bounding rectangle of the pickArea.

When the selection is done ( QwtPlotPicker::selected(const QRectF&) ) simply attach a QwtPlotZoneItem according to the x coordinates of the selected rectangle.

Uwe