diff --git a/.github/workflows/asv.yml b/.github/workflows/asv.yml
index 89fb921..875ae67 100644
--- a/.github/workflows/asv.yml
+++ b/.github/workflows/asv.yml
@@ -18,7 +18,7 @@ jobs:
         os: [ubuntu-latest, windows-latest, macos-latest, ubuntu-24.04-arm]
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
 
@@ -61,7 +61,7 @@ jobs:
       url: ${{ steps.deployment.outputs.page_url }}
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
 
@@ -70,7 +70,7 @@ jobs:
         run: git branch main origin/main
 
       - name: Prepare previous ASV results
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         continue-on-error: true
         with:
           ref: gh-pages
diff --git a/.github/workflows/emscripten.yml b/.github/workflows/emscripten.yml
index 3cb6852..bca977b 100644
--- a/.github/workflows/emscripten.yml
+++ b/.github/workflows/emscripten.yml
@@ -7,7 +7,7 @@ jobs:
   build-wasm-wheel:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Build WASM wheel
         uses: pypa/cibuildwheel@v3.3.0
         env:
diff --git a/.github/workflows/static.yml b/.github/workflows/static.yml
index 98e3c8d..f1e50bc 100644
--- a/.github/workflows/static.yml
+++ b/.github/workflows/static.yml
@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: prefix-dev/setup-pixi@v0.9.3
         with:
           environments: static
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index c5d1b3f..f39223f 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -15,7 +15,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: prefix-dev/setup-pixi@v0.9.3
         with:
           environments: >-
@@ -39,8 +39,8 @@ jobs:
       - name: Test with doctest
         if: matrix.os == 'ubuntu-latest'
         run: |
-          pixi run -e docs doc
-          pixi run -e docs doc sphinx-build doctest
+          pixi run -e docs doc-build
+          pixi run -e docs doc-build sphinx-build doctest
       - name: Test nogil
         run: |
           pixi run -e nogil nogil-eta
diff --git a/.github/workflows/wheel.yml b/.github/workflows/wheel.yml
index 457d38d..52a022f 100644
--- a/.github/workflows/wheel.yml
+++ b/.github/workflows/wheel.yml
@@ -7,7 +7,7 @@ jobs:
   build-sdist:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: prefix-dev/setup-pixi@v0.9.3
         with:
           environments: dev
@@ -31,7 +31,7 @@ jobs:
         os: [ubuntu-latest, windows-latest, macos-latest, ubuntu-24.04-arm]
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Build wheels
         uses: pypa/cibuildwheel@v3.3.0
         env:
diff --git a/examples/plot_narx_msa.py b/examples/plot_narx_msa.py
index 75b085e..c9dbf7f 100644
--- a/examples/plot_narx_msa.py
+++ b/examples/plot_narx_msa.py
@@ -24,7 +24,7 @@
 # where :math:`y` is the output signal and :math:`u` is the input signal, which is
 # :math:`u(t) = 2.5\cos(2\pi t)`.
 #
-# The phase portraits of the Duffing equation are shown below.
+# The phase portraits and the vector field of the Duffing equation are shown below.
 
 import matplotlib.pyplot as plt
 import numpy as np
@@ -67,10 +67,35 @@ def auto_duffing_equation(y, t):
 for i in range(n_init):
     sol[i] = odeint(auto_duffing_equation, y0[i], t)
 
+# Phase portraits
 for i in range(n_init):
     plt.plot(sol[i, :, 0], sol[i, :, 1], c="tab:blue")
 
-plt.title("Phase portraits of Duffing equation")
+# Vector field
+y_min = np.min(sol[:, :, 0])-0.2
+y_max = np.max(sol[:, :, 0])+0.2
+dot_y_min = np.min(sol[:, :, 1])-0.2
+dot_y_max = np.max(sol[:, :, 1])+0.2
+y, dot_y = np.meshgrid(
+    np.linspace(y_min, y_max, 30), np.linspace(dot_y_min, dot_y_max, 30)
+)
+ddot_y = auto_duffing_equation([y, dot_y], 0)[1]
+plt.streamplot(
+    y,
+    dot_y,
+    dot_y,
+    ddot_y,
+    color=(0.5, 0.5, 0.5, 0.3),
+    density=1.5,
+    minlength=0.02,
+    maxlength=0.1,
+    linewidth=0.5,
+    arrowsize=0.5,
+)
+
+plt.xlim(y_min, y_max)
+plt.ylim(dot_y_min, dot_y_max)
+plt.title("Phase portraits and vector field of Duffing equation")
 plt.xlabel("y(t)")
 plt.ylabel("dy/dt(t)")
 plt.show()
diff --git a/pixi.toml b/pixi.toml
index d424c74..42e77dc 100644
--- a/pixi.toml
+++ b/pixi.toml
@@ -113,7 +113,7 @@ type = { cmd = "mypy . --ignore-missing-imports", cwd = "fastcan" }
 spell = "codespell fastcan"
 
 [feature.docs.tasks]
-doc = { cmd = "{{ SPHINXBUILD }} -M {{ CMD }} {{ SOURCEDIR }} {{ BUILDDIR }} {{ SPHINXOPTS }} --fail-on-warning", cwd = "doc", args = [{ arg = "SPHINXBUILD", default = "sphinx-build" }, { arg = "CMD", default = "html" }, { arg = "SOURCEDIR", default = "." }, { arg = "BUILDDIR", default = "_build" }, { arg = "SPHINXOPTS", default = "" }] }
+doc-build = { cmd = "{{ SPHINXBUILD }} -M {{ CMD }} {{ SOURCEDIR }} {{ BUILDDIR }} {{ SPHINXOPTS }} --fail-on-warning", cwd = "doc", args = [{ arg = "SPHINXBUILD", default = "sphinx-build" }, { arg = "CMD", default = "html" }, { arg = "SOURCEDIR", default = "." }, { arg = "BUILDDIR", default = "_build" }, { arg = "SPHINXOPTS", default = "" }] }
 doc-clean = { cmd = "rm -rf {{ BUILDDIR }} generated auto_examples jupyterlite_contents .jupyterlite.doit.db _contents _output .cache", cwd = "doc", args = [{ arg = "BUILDDIR", default = "_build" }] }
 doc-deploy = { cmd = "python -m http.server" , cwd = "doc/_build/html" }
 doc-plantuml = { cmd = "plantuml -tsvg {{ SOURCE }} -o {{ OUTPUT }}", cwd = "doc", args = [{ arg = "SOURCE", default = "diagram.puml" }, { arg = "OUTPUT", default = "_build" }] }