Man pages matching name qwtalphacolormap in section 3:

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