Man pages matching name qwtplotcanvas in section 3:

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