Man pages matching name qwtsymbol in section 3:

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