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
KDE github flatpak: https://github.com/KDE/flatpak-kde-applications