'qgis', one of the few 'qwt' dependees in nixpkgs, fails to build with qwt 6. So I'm not moving the default version away from 5.x. Also, not changing the default allows easy/safe cherry-picking to the stable branch.
Nixpkgs is a collection of packages for Nix package manager. Nixpkgs also includes NixOS linux distribution source code.