QwtPlot is a composite widget, what makes certain things easier and other things harder.
For your particular use case you have the problem, that the line has to painted on the plot canvas, while the label has to be painted on the scale widget.
The line on the canvas would simply be a QwtPlotMarker - note, that such a marker could consist of a line and a text label ( see the bode example ).
Putting some labels on top of the scale is not supported out of the box, but can be implemented without too much work by overloading QwtScaleDraw::draw.
Uwe
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