The release tarballs for version 2.6 (with no patch version) have been placed in a download directory /2.6.0./ (with patch version). The tarball itself has 2.6 (no patch version) in the name. The CMakeLists says -DLVERSION="2.6.0" . Please be consistent with the versioning -- either major.minor.patch everywhere, or nowhere. That makes it much easier for existing tooling and packaging to find the release.
Thanks for the pointer. The tarball renamed to labplot-2.6.0.tar.xz.