Man pages matching name qwtplaintextengine in section 3:

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