Comment # 1 on bug 1032028 from Wolfgang Bauer

qwt6 is no longer maintained in KDE:Distro:Factory, but rather in
devel:languages:c++ now (and not by the KDE team anymore).

There is a seaparate qwt6-qt5 package meanwhile though (since May 2017) and
they should be co-installable AFAICS, so closing as fixed.