Man pages matching name qwtplotspectrogram in section 3:

QwtPlotSpectrogram.3.en	debian 10 / libqwt-doc
QwtPlotSpectrogram.3.en	debian 10 / libqwt5-doc
QwtPlotSpectrogram.3.en	debian 11 / libqwt-doc
QwtPlotSpectrogram.3.en	debian 12 / libqwt-doc