Hi,

The /usr/lib/x86_64-linux-gnu/pkgconfig/Qt6Qwt6.pc file for the
libqwt-qt6-dev package seems to be hardcoded to find components from Qt5,
not Qt6.

Configuring this CMake project in a Qt6-only Debian testing container
yields the following error:
=== CMakeLists.txt ===
cmake_minimum_required(VERSION 3.16)
project(qwt_test LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_AUTOMOC ON)

find_package(Qt6 REQUIRED COMPONENTS Widgets)

find_package(PkgConfig REQUIRED)
pkg_check_modules(QWT REQUIRED IMPORTED_TARGET Qt6Qwt6)

add_executable(qwt_test main.cpp)
target_link_libraries(qwt_test PRIVATE Qt6::Widgets PkgConfig::QWT)

=== Log output ===
-- Checking for module 'Qt6Qwt6'
--   Package 'Qt5Widgets', required by 'Qt6Qwt6', not found
Package 'Qt5Concurrent', required by 'Qt6Qwt6', not found
Package 'Qt5PrintSupport', required by 'Qt6Qwt6', not found
Package 'Qt5Svg', required by 'Qt6Qwt6', not found
Package 'Qt5OpenGL', required by 'Qt6Qwt6', not found
CMake Error at /usr/share/cmake-4.3/Modules/FindPkgConfig.cmake:1093
(message):
  The following required packages were not found:

   - Qt6Qwt6


The issue has been be fixed upstream after version 6.3:
https://sourceforge.net/p/qwt/git/ci/99a45521e96644a63c35353b22afd27499ded3af/

Other distros have patched the 6.3 version:
https://src.fedoraproject.org/rpms/qwt/blob/rawhide/f/qwt-pkgconfig.patch

Best regards,
Håkon Vågsether

We believe that the bug you reported is fixed in the latest version of
qwt, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1139598@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
A. Maitland Bottoms <bottoms@debian.org> (supplier of updated qwt package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Wed, 12 Aug 2026 23:42:46 -0400
Source: qwt
Architecture: source
Version: 6.3.0-4
Distribution: unstable
Urgency: medium
Maintainer: Gudjon I. Gudjonsson <gudjon@gudjon.org>
Changed-By: A. Maitland Bottoms <bottoms@debian.org>
Closes: 1139598
Changes:
 qwt (6.3.0-4) unstable; urgency=medium
 .
   * Add Debian Qt/KDE Maintainers to Uploaders for better team visibility
   * Include some upstream post-6.3.0 bugfixes:
     compiler error when qreal is a float fixed,
     QwtPlotScaleItem update bug fixed when changing,
     pkgconfig for Qt 5-6 adjustments, (Closes: #1139598)
     QwtPlotRenderer FrameWithScale rounding error,
     IWYU pragma,
     rpath fixed opengl setting fixed in playground.pri,
     avoid QCheckBox stateChanged for Qt 6.7
   * Bump standards version to 4.7.4. No changes needed
   * libqwt-headers: set Architecture: all as suggested by the multiarch hinter
   * Add hardening flags like other Qt components
   * Update watch file to version 5
   * Update symbols files
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 77c4e957cc0d125262d95711e70dfca6b0ae072f 46084 qwt_6.3.0-4.debian.tar.xz
 e2b6884352254daf6418cf9275743624b53384fd 14608 qwt_6.3.0-4_amd64.buildinfo
Checksums-Sha256:
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 52f4eca33de975532171479a34f86e8aba9f405772237027fb130605e20145d1 46084 qwt_6.3.0-4.debian.tar.xz
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