You can't attach a marker to a curve, but you can set the position of the marker to something that is related to the coordinates of the curve points. Instead you can use the legend what is the usual concept for showing a textual information about a curve.

If you want to "attach" some text according to a line ( rotated in direction of this line with some pixels distance ) you have to derive from QwtPlotCurve overloading the draw method. But please don't ask for an algorithm how to align a text label to a polygon ( not only a line ) - I don't have one.

Uwe