const int spectrWidth = 1000;
const int spectrHeight = 500;
QPixmap spectrPixmap(spectrWidth, spectrHeight);
QPainter p(&spectrPixmap);

for (int ir = 0; ir < spectrHeight; ir++)
{
    for (int ic = 0; ic < spectrWidth; ic++)
    {
        double data = getDataForCoordinates(ic, ir); // your function
        QColor color = getColorForData(data);  // your function
        p.setPen(color);
        p.drawPoint(ic, ir);
    }
}