[ci] add matplotlib

.github/workflows/tests.yml

@@ -29,7 +29,7 @@ jobs:
       run: python -m pip install .
 
     - name: Install test dependencies
-      run: python -m pip install numpy cython setuptools scipy flake8 pycodestyle pydocstyle pytest coverage 'coverage_lcov[extras] @ git+https://github.com/JohannesBuchner/coverage-lcov@feature-coverage7'
+      run: python -m pip install matplotlib numpy cython setuptools scipy flake8 pycodestyle pydocstyle pytest coverage 'coverage_lcov[extras] @ git+https://github.com/JohannesBuchner/coverage-lcov@feature-coverage7'
 
     - name: Install local build dependencies
       run: python -m pip install .