Canvas never transparent

[Spitfire]

It's quite simple - when you already know it

plot->canvas()->setPaintAttribute( QwtPlotCanvas::PaintPacked, false );
plot->canvas()->setPaintAttribute( QwtPlotCanvas::PaintCached, false );
plot->setCanvasBackground( Qt::transparent );

plot->canvas()->setPaintAttribute( QwtPlotCanvas::PaintPacked, false );
plot->canvas()->setPaintAttribute( QwtPlotCanvas::PaintCached, false );
plot->setCanvasBackground( Qt::transparent );

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Using this method you may get severe performance hit with large plots as caching is disabled, but I'm not sure if there's another way of doing it.