Man pages matching name qwtplotmarker in section 3:

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