diff --git a/explorer.py b/explorer.py
index ea94ad5..b1955de 100644
--- a/explorer.py
+++ b/explorer.py
@@ -485,6 +485,77 @@ def load_party_axis_scores(db_path: str) -> Dict[str, List[float]]:
             pass
 
 
+@st.cache_data(show_spinner="Partij-MP vectoren laden…")
+def load_party_mp_vectors(db_path: str) -> Dict[str, List[np.ndarray]]:
+    """Return per-party lists of individual MP SVD vectors.
+
+    Same MP→party mapping as load_party_axis_scores(), but returns the raw
+    per-MP vectors instead of averaging them. Suitable for bootstrap CI
+    computation.
+
+    Returns:
+        {party_name: [np.ndarray(50,), ...]}  — one array per MP.
+    """
+    try:
+        con = duckdb.connect(database=db_path, read_only=True)
+
+        # Build mp → party mapping (same logic as load_party_axis_scores)
+        meta_ordered = con.execute(
+            "SELECT mp_name, party FROM mp_metadata "
+            "WHERE van >= '2023-11-22' OR tot_en_met IS NULL OR tot_en_met >= '2023-11-22' "
+            "ORDER BY van ASC"
+        ).fetchall()
+        mp_party: Dict[str, str] = {}
+        for mp_name, party in meta_ordered:
+            if mp_name and party:
+                mp_party[mp_name] = _PARTY_NORMALIZE.get(party, party)
+
+        # Individual MP vectors from current_parliament
+        rows = con.execute(
+            "SELECT entity_id, vector FROM svd_vectors "
+            "WHERE entity_type='mp' AND window_id='current_parliament'"
+        ).fetchall()
+
+        party_vecs: Dict[str, List[np.ndarray]] = {}
+        for entity_id, raw_vec in rows:
+            party = mp_party.get(entity_id)
+            if party is None or party not in CURRENT_PARLIAMENT_PARTIES:
+                continue
+            if isinstance(raw_vec, str):
+                vec = json.loads(raw_vec)
+            elif isinstance(raw_vec, (bytes, bytearray)):
+                vec = json.loads(raw_vec.decode())
+            elif isinstance(raw_vec, list):
+                vec = raw_vec
+            else:
+                try:
+                    vec = list(raw_vec)
+                except Exception:
+                    continue
+            fvec = np.array([float(v) if v is not None else 0.0 for v in vec])
+            party_vecs.setdefault(party, []).append(fvec)
+
+        return party_vecs
+    except Exception:
+        logger.exception("Failed to load party MP vectors")
+        return {}
+    finally:
+        try:
+            con.close()
+        except Exception:
+            pass
+
+
+@st.cache_data(show_spinner="Bootstrap CI berekenen…")
+def _cached_bootstrap_cis(
+    _party_mp_vectors: Dict[str, List[np.ndarray]],
+) -> Dict[str, Dict]:
+    """Thin caching wrapper around compute_party_bootstrap_cis."""
+    from analysis.political_axis import compute_party_bootstrap_cis
+
+    return compute_party_bootstrap_cis(_party_mp_vectors)
+
+
 @st.cache_data(show_spinner="Scree-plot laden…")
 def load_scree_data(db_path: str) -> List[float]:
     """Return explained variance ratios (%) for all SVD components, sorted descending.
@@ -621,18 +692,28 @@ def _render_scree_plot(importances: List[float], n_show: int = 15) -> None:
     st.plotly_chart(fig, use_container_width=True)
 
 
-def _render_party_axis_chart(
-    party_scores: Dict[str, List[float]], comp_sel: int, theme: dict
-) -> None:
-    """Render a 1D horizontal Plotly scatter of party positions on SVD axis `comp_sel`.
+def _build_party_axis_figure(
+    party_scores: Dict[str, List[float]],
+    comp_sel: int,
+    theme: dict,
+    bootstrap_data: Optional[Dict[str, Dict]] = None,
+) -> Optional[go.Figure]:
+    """Build a 1D horizontal Plotly scatter of party positions on SVD axis `comp_sel`.
 
-    Each party is plotted at its score on a single horizontal axis (y=0).
-    When theme['flip'] is True the scores are negated so that the progressive/left
-    side always appears on the left of the chart.
+    Pure function that returns a go.Figure (no Streamlit calls).
+
+    Args:
+        party_scores: {party_name: [float*k]} — mean SVD vectors per party.
+        comp_sel: 1-indexed SVD axis number.
+        theme: dict with keys label, explanation, positive_pole, negative_pole, flip.
+        bootstrap_data: optional output from compute_party_bootstrap_cis —
+            {party: {centroid, ci_lower, ci_upper, std, n_mps}}.
+
+    Returns:
+        go.Figure, or None if no data available.
     """
     if not party_scores:
-        st.caption("_Partijdata niet beschikbaar voor deze as._")
-        return
+        return None
 
     axis_idx = comp_sel - 1  # 0-based index into the 50-dim vector
     flip = theme.get("flip", False)
@@ -645,13 +726,21 @@ def _render_party_axis_chart(
             data.append({"party": party, "score": score})
 
     if not data:
-        st.caption("_Geen partijscores voor deze as._")
-        return
+        return None
 
     scores = [d["score"] for d in data]
     parties = [d["party"] for d in data]
     colours = [PARTY_COLOURS.get(p, "#9E9E9E") for p in parties]
-    hover = [f"{p}: {s:.3f}" for p, s in zip(parties, scores)]
+
+    # Build hover text: include N when bootstrap data available
+    if bootstrap_data:
+        hover = []
+        for p, s in zip(parties, scores):
+            bd = bootstrap_data.get(p)
+            n_mps = bd["n_mps"] if bd else "?"
+            hover.append(f"{p}: {s:.3f} (N={n_mps})")
+    else:
+        hover = [f"{p}: {s:.3f}" for p, s in zip(parties, scores)]
 
     # Determine axis labels: left = progressive pole, right = conservative pole
     pos_pole = theme.get("positive_pole", "")
@@ -674,20 +763,43 @@ def _render_party_axis_chart(
             showlegend=False,
         )
     )
+
+    # Build marker kwargs — bootstrap data adds error bars and diamond markers
+    marker_kwargs: dict = {"size": 18, "color": colours}
+    error_x_kwargs: Optional[dict] = None
+
+    if bootstrap_data:
+        error_array = []
+        symbols = []
+        for p in parties:
+            bd = bootstrap_data.get(p)
+            if bd:
+                err = (bd["ci_upper"][axis_idx] - bd["ci_lower"][axis_idx]) / 2
+                error_array.append(abs(float(err)))
+                symbols.append("diamond" if bd["n_mps"] == 1 else "circle")
+            else:
+                error_array.append(0.0)
+                symbols.append("circle")
+        marker_kwargs["symbol"] = symbols
+        error_x_kwargs = {"type": "data", "array": error_array, "visible": True}
+
     # Party markers
-    fig.add_trace(
-        go.Scatter(
-            x=scores,
-            y=[0] * len(scores),
-            mode="markers+text",
-            text=parties,
-            textposition="top center",
-            marker={"size": 18, "color": colours},
-            hovertext=hover,
-            hoverinfo="text",
-            showlegend=False,
-        )
-    )
+    scatter_kwargs: dict = {
+        "x": scores,
+        "y": [0] * len(scores),
+        "mode": "markers+text",
+        "text": parties,
+        "textposition": "top center",
+        "marker": marker_kwargs,
+        "hovertext": hover,
+        "hoverinfo": "text",
+        "showlegend": False,
+    }
+    if error_x_kwargs is not None:
+        scatter_kwargs["error_x"] = error_x_kwargs
+
+    fig.add_trace(go.Scatter(**scatter_kwargs))
+
     fig.update_layout(
         height=160,
         margin={"l": 10, "r": 10, "t": 10, "b": 30},
@@ -702,6 +814,24 @@ def _render_party_axis_chart(
         plot_bgcolor="rgba(0,0,0,0)",
         paper_bgcolor="rgba(0,0,0,0)",
     )
+    return fig
+
+
+def _render_party_axis_chart(
+    party_scores: Dict[str, List[float]],
+    comp_sel: int,
+    theme: dict,
+    bootstrap_data: Optional[Dict[str, Dict]] = None,
+) -> None:
+    """Render a 1D horizontal Plotly scatter of party positions on SVD axis `comp_sel`.
+
+    Delegates figure construction to _build_party_axis_figure, then renders via
+    st.plotly_chart.
+    """
+    fig = _build_party_axis_figure(party_scores, comp_sel, theme, bootstrap_data)
+    if fig is None:
+        st.caption("_Partijdata niet beschikbaar voor deze as._")
+        return
     st.plotly_chart(fig, use_container_width=True)
 
 
@@ -1648,7 +1778,13 @@ def build_svd_components_tab(db_path: str) -> None:
 
     # Party axis chart
     party_scores = load_party_axis_scores(db_path)
-    _render_party_axis_chart(party_scores, comp_sel, theme)
+    party_mp_vectors = load_party_mp_vectors(db_path)
+    bootstrap_data = (
+        _cached_bootstrap_cis(party_mp_vectors) if party_mp_vectors else None
+    )
+    _render_party_axis_chart(
+        party_scores, comp_sel, theme, bootstrap_data=bootstrap_data
+    )
 
     # Batch-fetch motion details (title, date, policy_area, url, body_text, voting_results)
     motion_ids = [m.get("motion_id") for m in motions if m.get("motion_id") is not None]
diff --git a/tests/test_explorer_chart.py b/tests/test_explorer_chart.py
new file mode 100644
index 0000000..c1629b1
--- /dev/null
+++ b/tests/test_explorer_chart.py
@@ -0,0 +1,175 @@
+"""Tests for _build_party_axis_figure and load_party_mp_vectors in explorer.py."""
+
+import numpy as np
+import plotly.graph_objects as go
+import pytest
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def _make_party_scores(n_parties=3, dim=50):
+    """Return a minimal party_scores dict for testing."""
+    rng = np.random.default_rng(0)
+    names = [f"Party{i}" for i in range(n_parties)]
+    return {name: rng.standard_normal(dim).tolist() for name in names}
+
+
+def _make_theme(flip=False):
+    return {
+        "label": "Test axis",
+        "explanation": "A test axis.",
+        "positive_pole": "Left",
+        "negative_pole": "Right",
+        "flip": flip,
+    }
+
+
+def _make_bootstrap_data(party_scores, dim=50):
+    """Build synthetic bootstrap_data matching party_scores keys.
+
+    Party0 gets n_mps=1 (single-MP party → diamond marker).
+    Others get n_mps > 1 with a real CI spread.
+    """
+    rng = np.random.default_rng(1)
+    result = {}
+    for i, party in enumerate(party_scores):
+        centroid = np.array(party_scores[party])
+        if i == 0:
+            # Single-MP party
+            result[party] = {
+                "centroid": centroid,
+                "ci_lower": centroid.copy(),
+                "ci_upper": centroid.copy(),
+                "std": np.zeros(dim),
+                "n_mps": 1,
+            }
+        else:
+            spread = rng.uniform(0.01, 0.05, size=dim)
+            result[party] = {
+                "centroid": centroid,
+                "ci_lower": centroid - spread,
+                "ci_upper": centroid + spread,
+                "std": spread / 2,
+                "n_mps": 5 + i,
+            }
+    return result
+
+
+# ---------------------------------------------------------------------------
+# Tests
+# ---------------------------------------------------------------------------
+
+
+class TestBuildPartyAxisFigure:
+    """Tests for _build_party_axis_figure (pure Plotly figure construction)."""
+
+    def test_returns_figure_without_bootstrap(self):
+        """Basic call without bootstrap → returns go.Figure with 2 traces."""
+        from explorer import _build_party_axis_figure
+
+        party_scores = _make_party_scores()
+        theme = _make_theme()
+        fig = _build_party_axis_figure(party_scores, comp_sel=1, theme=theme)
+
+        assert isinstance(fig, go.Figure)
+        assert len(fig.data) == 2  # baseline + markers
+        # First trace is the baseline line
+        assert fig.data[0].mode == "lines"
+        # Second trace is the marker scatter
+        assert "markers" in fig.data[1].mode
+
+    def test_returns_none_for_empty_scores(self):
+        """Empty party_scores returns None (no figure)."""
+        from explorer import _build_party_axis_figure
+
+        fig = _build_party_axis_figure({}, comp_sel=1, theme=_make_theme())
+        assert fig is None
+
+    def test_with_bootstrap_has_error_x_and_diamonds(self):
+        """Call WITH bootstrap_data → error_x on marker trace, diamond for N=1."""
+        from explorer import _build_party_axis_figure
+
+        party_scores = _make_party_scores()
+        theme = _make_theme()
+        bootstrap_data = _make_bootstrap_data(party_scores)
+        fig = _build_party_axis_figure(
+            party_scores, comp_sel=1, theme=theme, bootstrap_data=bootstrap_data
+        )
+
+        assert isinstance(fig, go.Figure)
+        assert len(fig.data) == 2
+
+        marker_trace = fig.data[1]
+
+        # error_x should be present and visible
+        assert marker_trace.error_x is not None
+        assert marker_trace.error_x.visible is True
+        assert marker_trace.error_x.type == "data"
+        assert len(marker_trace.error_x.array) == 3  # 3 parties
+
+        # All error bar values should be non-negative
+        for err in marker_trace.error_x.array:
+            assert err >= 0.0
+
+        # Marker symbols: first party (N=1) → diamond, others → circle
+        symbols = list(marker_trace.marker.symbol)
+        assert symbols[0] == "diamond"
+        assert all(s == "circle" for s in symbols[1:])
+
+    def test_with_bootstrap_hover_includes_n(self):
+        """Hover text includes N=<count> for each party."""
+        from explorer import _build_party_axis_figure
+
+        party_scores = _make_party_scores()
+        theme = _make_theme()
+        bootstrap_data = _make_bootstrap_data(party_scores)
+        fig = _build_party_axis_figure(
+            party_scores, comp_sel=1, theme=theme, bootstrap_data=bootstrap_data
+        )
+
+        marker_trace = fig.data[1]
+        for ht in marker_trace.hovertext:
+            assert "(N=" in ht
+
+    def test_flip_negates_scores_but_error_bars_stay_positive(self):
+        """When flip=True, scores are negated but error bar magnitudes stay positive."""
+        from explorer import _build_party_axis_figure
+
+        party_scores = _make_party_scores()
+        theme_no_flip = _make_theme(flip=False)
+        theme_flip = _make_theme(flip=True)
+        bootstrap_data = _make_bootstrap_data(party_scores)
+
+        fig_normal = _build_party_axis_figure(
+            party_scores, comp_sel=1, theme=theme_no_flip, bootstrap_data=bootstrap_data
+        )
+        fig_flipped = _build_party_axis_figure(
+            party_scores, comp_sel=1, theme=theme_flip, bootstrap_data=bootstrap_data
+        )
+
+        normal_scores = list(fig_normal.data[1].x)
+        flipped_scores = list(fig_flipped.data[1].x)
+
+        # Scores should be negated
+        for ns, fs in zip(normal_scores, flipped_scores):
+            assert pytest.approx(ns) == -fs
+
+        # Error bars should be the same (positive) in both cases
+        normal_errors = list(fig_normal.data[1].error_x.array)
+        flipped_errors = list(fig_flipped.data[1].error_x.array)
+        for ne, fe in zip(normal_errors, flipped_errors):
+            assert ne >= 0.0
+            assert fe >= 0.0
+            assert pytest.approx(ne) == fe
+
+
+class TestLoadPartyMpVectorsImportable:
+    """Smoke test: verify load_party_mp_vectors is importable."""
+
+    def test_importable(self):
+        from explorer import load_party_mp_vectors
+
+        assert callable(load_party_mp_vectors)