Okay, it seems I've found the answer only minutes after posting the question (and after hours of having no clue before ;)):

The problem is QwtPlotCurve::minYValue/maxYValue or the boundingRect respectively. Those are seemingly only updated on calls to "setRawSamples", but not when the underlying data changes or replot is called.

If anyone has a better solution for me (other than changing the underlying data to directly feed it into the QwtPlotCurve), please let me know!