Man pages matching name qwtplotmagnifier in section 3:

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