Man pages matching name qwtlegend in section 3:

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