Man pages matching name qwtcurvefitter in section 3:

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