Binary Factory already creates flatpak: https://binary-factory.kde.org/job/Labplot2_x86_64_flatpak/
But it's not on flathub yet.
Labplot2_x86_64_flatpak puts package to http://distribute.kde.org/flatpak-apps-testing/
Docu: https://community.kde.org/Guidelines_and_HOWTOs/Flatpak, https://github.com/flathub/flathub/blob/master/CONTRIBUTING.md
KDE github flatpak: https://github.com/KDE/flatpak-kde-applications
TODO:
* Test flatpak created by Binary Factory
* Add dependency to manifest on Binary Factory and check build/package
* Upload manifest to Flathub