How to change axis scale with out changing data series?

[Uwe]

The conversion from FFT bin to frequency is a simple scale factor

Ok, lets call this factor 'f'.

I am not sure I have understood your suggestions therefore.

Assuming you have your values in m_data, what is an array of doubles:

virtual QPointF YourSeriesData::sample(size_t i) const
{
    return QPointF( f * toFFTbin(i), m_data[i]); // do your scaling here
}

virtual QPointF YourSeriesData::sample(size_t i) const
{
    return QPointF( f * toFFTbin(i), m_data[i]); // do your scaling here
}

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Uwe