QwtTransform is for the mapping between the axis and widget coordinate systems, but you are looking for mapping the x coordinates of your samples into the coordinate system of your x axis - that's a different one !
Don't know about the details of your project, but to me it looks like you should set up your plot coordinate system based on energy. The mapping of your samples would be done then by overloading methods of QwtSeriesData<QPointF> to return energy.

Uwe