Man pages matching name qwtpainter in section 3:

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