Align spectrum with frequency scale

[srcejon]

At low FFT sizes, it is apparent that the spectrum is not quite aligned with the frequency scale. This is most obvious when peak markers / table are enabled, as the markers don't align with the peaks on the right hand side of the spectrum. (At high FFT sizes, the difference is ~1 pixel, so not visible). But it's also noticeable if you place a marker on a point, and the frequency isn't quite what you'd calculate for the centre of the bin.

This is because the frequency scale goes from -Fs/2/size to Fs/2/size (so size+1 points), but the spectrum line is drawn with size points. The code currently stretches the spectrum over the frequency range, by doing (m_nbBins - 1):

    m_glHistogramSpectrumMatrix.scale(
        ((float) 2 * (width() - m_leftMargin - m_rightMargin)) / ((float) width() * (float)(m_nbBins - 1)),
        ((float) 2*m_histogramHeight / height()) / m_powerRange

If we change this to m_nbBins, the markers align with the peaks, but we are left with a gap on the right hand side of the spectrum:

To draw up to the end of the scale, we can replicate the Nyquist point from the FFT (i.e the point at N/2 in the FFT output). Currently, we just draw this as the lowest frequency point, but it actually can be considered to be a positive frequency as well, so it can be drawn at both ends of the spectrum, giving us the extra point we need.

For a description of this, see: https://dsp.stackexchange.com/questions/72515/in-the-context-of-dft-where-does-the-nyquist-frequency-sample-belong-in-a-doubl and the FFTshift section in: https://www.gaussianwaves.com/2015/11/interpreting-fft-results-complex-dft-frequency-bins-and-fftshift/

This then gives a correctly aligned spectrum that fills the full frequency range: