ci: Add matplotlib dependency

Author: jorisv

.github/workflows/ubuntu.yml

@@ -59,7 +59,6 @@ jobs:
         uses: actions/cache@v5
         with:
           path: .ccache
-          save-always: true
           key: ccache-ubuntu-${{ matrix.container }}-${{ github.sha }}
           restore-keys: ccache-ubuntu-${{ matrix.container }}-
 
@@ -83,6 +82,7 @@ jobs:
                                    libmimalloc-dev \
                                    python3-numpy \
                                    python3-pytest \
+                                   python3-matplotlib \
                                    robotpkg-example-robot-data \
                                    robotpkg-py${PYTHON3_VERSION}-example-robot-data \
                                    robotpkg-py${PYTHON3_VERSION}-eigenpy \
@@ -108,6 +108,7 @@ jobs:
                                    catch2 \
                                    python3-numpy \
                                    python3-pytest \
+                                   python3-matplotlib \
                                    robotpkg-example-robot-data \
                                    robotpkg-py${PYTHON3_VERSION}-example-robot-data \
                                    robotpkg-py${PYTHON3_VERSION}-eigenpy \