diff --git a/src/ariadnepy/plot/_custom.py b/src/ariadnepy/plot/_custom.py
index 6676796..4d6eeaa 100644
--- a/src/ariadnepy/plot/_custom.py
+++ b/src/ariadnepy/plot/_custom.py
@@ -86,6 +86,15 @@ def add_resource(
         df = df.iloc[:, :2]
     from_col, to_col = df.columns[0], df.columns[1]
 
+    # Validate that at least one feature is already in the graph (mirrors R's addResource check).
+    existing = set(graph.vs["name"]) if graph.vcount() > 0 else set()
+    new_vars = {col for col in (from_col, to_col) if col not in existing}
+    if len(new_vars) == 2:
+        raise AriadneError(
+            f"At least one feature must already be in the graph. "
+            f"Neither {from_col!r} nor {to_col!r} was found."
+        )
+
     # Cache the linkmap as parquet
     cache_dir = init_cache()
     safe = "".join(c if c.isalnum() else "_" for c in str(file))[:80]
@@ -105,15 +114,23 @@ def add_resource(
     graph = graph.copy()
 
     # Add vertices if missing
-    existing = set(graph.vs["name"]) if graph.vcount() > 0 else set()
     for node in (from_col, to_col):
         if node not in existing:
             graph.add_vertex(name=node)
             existing.add(node)
 
-    # Add edge
+    # Check for duplicate edge (same node pair + same res_name) before adding.
     src_idx = graph.vs.find(name=from_col).index
     tgt_idx = graph.vs.find(name=to_col).index
+    if not force:
+        for e in graph.es:
+            pair = {graph.vs[e.source]["name"], graph.vs[e.target]["name"]}
+            if pair == {from_col, to_col} and e.attributes().get("source") == res_name:
+                raise AriadneError(
+                    f"A '{res_name}' edge between {from_col!r} and {to_col!r} already exists. "
+                    "Set force=True to overwrite it."
+                )
+
     graph.add_edge(src_idx, tgt_idx)
     for k, v in edge_attrs.items():
         graph.es[-1][k] = v
diff --git a/src/ariadnepy/plot/_draw.py b/src/ariadnepy/plot/_draw.py
index 5e39e6e..44c899c 100644
--- a/src/ariadnepy/plot/_draw.py
+++ b/src/ariadnepy/plot/_draw.py
@@ -75,7 +75,8 @@ def plot_path(
             path_edges.append((row["from"], row["to"]))
             path_edge_labels[(row["from"], row["to"])] = row.get("source", "")
 
-    path_edge_set = set(path_edges)
+    # Use frozensets so edge lookup is direction-agnostic, matching R's sort(c(from,to)) approach.
+    path_edge_set = {frozenset(e) for e in path_edges}
 
     # prune/focus both restrict layout to the path subgraph.
     if path_nodes and (focus or prune):
@@ -88,8 +89,8 @@ def plot_path(
     else:
         draw_graph = graph
 
-    # Fruchterman-Reingold layout — equivalent to R ariadne's "stress" layout
-    layout = draw_graph.layout("fr")
+    # Kamada-Kawai layout — closest available equivalent to R ariadne's "stress" layout
+    layout = draw_graph.layout("kk")
     all_names = draw_graph.vs["name"]
     pos = {name: tuple(layout[i]) for i, name in enumerate(all_names)}
 
@@ -111,7 +112,7 @@ def plot_path(
     for u, v, src in all_edges_raw:
         if u not in pos or v not in pos:
             continue
-        is_path_edge = (u, v) in path_edge_set or (v, u) in path_edge_set
+        is_path_edge = frozenset((u, v)) in path_edge_set
         color = _C_EDGE_PATH if is_path_edge else _C_EDGE_BG
         lw    = 2.0 if is_path_edge else 1.0
         x0, y0 = pos[u]
diff --git a/tests/test_core/test_graph/test_add_resource.py b/tests/test_core/test_graph/test_add_resource.py
new file mode 100644
index 0000000..52c1b99
--- /dev/null
+++ b/tests/test_core/test_graph/test_add_resource.py
@@ -0,0 +1,130 @@
+"""Tests for plot/_custom.py::add_resource.
+
+add_resource connects CSV *column names* as graph nodes (not row values).
+The fixture CSV has columns 'known_node' and 'new_node', so those two strings
+become the candidate vertex names.  The base graph below contains 'known_node'
+as a pre-existing vertex so that the "at least one feature must already be in
+the graph" invariant is satisfied for all positive-path tests.
+
+All tests use a synthetic offline undirected graph — no network calls.
+"""
+from __future__ import annotations
+
+import igraph as ig
+import pandas as pd
+import pytest
+
+from ariadnepy.exceptions import AriadneError
+from ariadnepy.plot._custom import add_resource
+
+
+# ── Fixtures ──────────────────────────────────────────────────────────────────
+
+
+@pytest.fixture
+def base_graph() -> ig.Graph:
+    """Minimal undirected graph that already contains 'known_node'.
+
+    Having 'known_node' pre-existing means a CSV whose first column is named
+    'known_node' will pass the "at least one feature in graph" guard.
+    """
+    g = ig.Graph(directed=False)
+    g.add_vertex(name="known_node")
+    return g
+
+
+@pytest.fixture
+def tmp_linkmap(tmp_path) -> str:
+    """CSV with columns 'known_node' and 'new_node' — two rows of sample data."""
+    df = pd.DataFrame({"known_node": ["x", "y"], "new_node": ["p", "q"]})
+    p = tmp_path / "map.csv"
+    df.to_csv(p, index=False)
+    return str(p)
+
+
+@pytest.fixture
+def both_new_linkmap(tmp_path) -> str:
+    """CSV whose column names ('alpha', 'beta') are both absent from the base graph.
+
+    Used to verify that add_resource raises AriadneError when neither column
+    name is already present as a graph vertex.
+    """
+    df = pd.DataFrame({"alpha": ["a", "b"], "beta": ["c", "d"]})
+    p = tmp_path / "both_new.csv"
+    df.to_csv(p, index=False)
+    return str(p)
+
+
+# ── Return-type test ──────────────────────────────────────────────────────────
+
+
+def test_add_resource_returns_igraph(base_graph, tmp_linkmap):
+    """add_resource must return an igraph.Graph instance."""
+    result = add_resource(base_graph, tmp_linkmap, res_name="TestDB")
+    assert isinstance(result, ig.Graph)
+
+
+# ── Edge addition tests ───────────────────────────────────────────────────────
+
+
+def test_add_resource_adds_new_edge(base_graph, tmp_linkmap):
+    """A new edge must be present in the returned graph."""
+    before = base_graph.ecount()
+    result = add_resource(base_graph, tmp_linkmap, res_name="TestDB")
+    assert result.ecount() == before + 1
+
+
+def test_add_resource_edge_has_source_attribute(base_graph, tmp_linkmap):
+    """The newly added edge must carry the 'source' attribute set to res_name."""
+    result = add_resource(base_graph, tmp_linkmap, res_name="TestDB")
+    assert result.es[-1]["source"] == "TestDB"
+
+
+# ── Node addition test ────────────────────────────────────────────────────────
+
+
+def test_add_resource_adds_new_node(base_graph, tmp_linkmap):
+    """The column name 'new_node' (absent from base_graph) must appear as a vertex."""
+    before_names = set(base_graph.vs["name"])
+    result = add_resource(base_graph, tmp_linkmap, res_name="TestDB")
+    after_names = set(result.vs["name"])
+    assert "new_node" in after_names - before_names
+
+
+# ── Validation-gap test — both columns new ───────────────────────────────────
+
+
+def test_add_resource_raises_when_both_columns_new(base_graph, both_new_linkmap):
+    """add_resource must raise AriadneError when neither CSV column name is in
+    the graph.
+
+    This guards against silently grafting a disconnected subgraph — mirroring
+    R's addResource check that at least one feature type must already exist.
+    """
+    with pytest.raises(AriadneError, match="At least one feature"):
+        add_resource(base_graph, both_new_linkmap, res_name="Disconnected")
+
+
+# ── One-column-known test ─────────────────────────────────────────────────────
+
+
+def test_add_resource_succeeds_when_one_column_known(base_graph, tmp_linkmap):
+    """add_resource must succeed (not raise) when one column name is already in
+    the graph as a vertex — 'known_node' is pre-existing in base_graph."""
+    result = add_resource(base_graph, tmp_linkmap, res_name="TestDB")
+    assert "known_node" in result.vs["name"]
+
+
+# ── Duplicate-edge guard (force=False) ───────────────────────────────────────
+
+
+def test_add_resource_force_false_raises_on_duplicate(base_graph, tmp_linkmap):
+    """Calling add_resource twice with force=False must raise AriadneError on the
+    second call to prevent silently creating duplicate edges for the same resource.
+
+    This test documents the intended validation behaviour; the implementation
+    must enforce it.
+    """
+    result = add_resource(base_graph, tmp_linkmap, res_name="TestDB", force=False)
+    with pytest.raises(AriadneError):
+        add_resource(result, tmp_linkmap, res_name="TestDB", force=False)
diff --git a/tests/test_core/test_graph/test_plot.py b/tests/test_core/test_graph/test_plot.py
new file mode 100644
index 0000000..455ac06
--- /dev/null
+++ b/tests/test_core/test_graph/test_plot.py
@@ -0,0 +1,108 @@
+"""Tests for plot/_draw.py::plot_path.
+
+All tests use a synthetic offline undirected graph — no network calls.
+Matplotlib is forced into the non-interactive Agg backend so the suite
+runs headlessly in CI.
+"""
+from __future__ import annotations
+
+import igraph as ig
+import matplotlib
+import pytest
+
+matplotlib.use("Agg")  # headless — must be set before importing pyplot
+
+from matplotlib.figure import Figure
+
+from ariadnepy.plot._draw import plot_path
+
+
+# ── Fixtures ──────────────────────────────────────────────────────────────────
+
+
+@pytest.fixture
+def simple_graph() -> ig.Graph:
+    """Minimal undirected graph: A -- B -- C with a single resource label."""
+    g = ig.Graph(directed=False)
+    for n in ("A", "B", "C"):
+        g.add_vertex(name=n)
+    g.add_edge(0, 1)
+    g.es[0]["source"] = "DB1"
+    g.add_edge(1, 2)
+    g.es[1]["source"] = "DB1"
+    return g
+
+
+# ── Return-type tests ─────────────────────────────────────────────────────────
+
+
+def test_plot_path_no_by_returns_figure(simple_graph):
+    """plot_path(graph) with no 'by' formula must return a matplotlib Figure."""
+    fig = plot_path(simple_graph)
+    assert isinstance(fig, Figure)
+
+
+def test_plot_path_by_k1_returns_figure(simple_graph):
+    """plot_path(graph, 'A ~ C', k=1) must return a matplotlib Figure."""
+    fig = plot_path(simple_graph, "A ~ C", k=1)
+    assert isinstance(fig, Figure)
+
+
+def test_plot_path_prune_returns_figure(simple_graph):
+    """plot_path with prune=True must return a matplotlib Figure."""
+    fig = plot_path(simple_graph, "A ~ C", k=1, prune=True)
+    assert isinstance(fig, Figure)
+
+
+def test_plot_path_focus_returns_figure(simple_graph):
+    """plot_path with focus=True must return a matplotlib Figure."""
+    fig = plot_path(simple_graph, "A ~ C", k=1, focus=True)
+    assert isinstance(fig, Figure)
+
+
+# ── Title tests ───────────────────────────────────────────────────────────────
+
+
+def test_plot_path_no_by_title_is_resource_graph(simple_graph):
+    """When no path formula is given the figure title must be 'Resource Graph'."""
+    fig = plot_path(simple_graph)
+    ax = fig.axes[0]
+    assert ax.get_title() == "Resource Graph"
+
+
+def test_plot_path_by_k2_title_contains_path_2(simple_graph):
+    """With k=2 the figure title must contain 'Path 2' (mirrors R plotPath behaviour)."""
+    # Build a branching graph so two distinct shortest paths exist.
+    g = ig.Graph(directed=False)
+    for n in ("A", "M1", "M2", "C"):
+        g.add_vertex(name=n)
+    g.add_edge(g.vs.find(name="A").index, g.vs.find(name="M1").index)
+    g.es[-1]["source"] = "DB1"
+    g.add_edge(g.vs.find(name="M1").index, g.vs.find(name="C").index)
+    g.es[-1]["source"] = "DB1"
+    g.add_edge(g.vs.find(name="A").index, g.vs.find(name="M2").index)
+    g.es[-1]["source"] = "DB1"
+    g.add_edge(g.vs.find(name="M2").index, g.vs.find(name="C").index)
+    g.es[-1]["source"] = "DB1"
+
+    fig = plot_path(g, "A ~ C", k=2)
+    ax = fig.axes[0]
+    assert "Path 2" in ax.get_title()
+
+
+# ── Validation tests ──────────────────────────────────────────────────────────
+
+
+def test_plot_path_prune_without_by_raises(simple_graph):
+    """prune=True without a 'by' formula must raise ValueError."""
+    with pytest.raises(ValueError, match="prune"):
+        plot_path(simple_graph, prune=True)
+
+
+# ── Axes structure tests ──────────────────────────────────────────────────────
+
+
+def test_plot_path_returns_exactly_one_axes(simple_graph):
+    """The returned Figure must contain exactly one Axes object."""
+    fig = plot_path(simple_graph)
+    assert len(fig.axes) == 1