How to change axis scale with out changing data series?

[Uwe]

Is this rather inefficient as the sample() member will be called 500 million times as opposed to 1024 times?

QwtSeriesData::sample() is QwtSeriesData::size() times called regardless what your axes ranges are or what values you return. In your example it should be 1024 for both situations.

Note, that you are returning points only. If you want to translate your points into a x1-x2, y representations, you need an item similar/derived from QwtPlotHistogram.

Uwe