The parameters x1 and x2 of QwtScaleEngine::autoScale method are in/out while the python bindings are generated as if they were only out. This prevents sub-classing QwtScaleEngine,QwtLinearScaleEngine,QwtLog10ScaleEngine in python-land.
My guess is that x1 and x2 must be explicitly marked as in/out in qwt_scale_engine.sip