fix: handle scalar string groups in ranked-gene plots

src/scanpy/plotting/legacy/_tools/__init__.py

@@ -406,6 +406,8 @@ def rank_genes_groups(  # noqa: PLR0912, PLR0913, PLR0915
 
     reference = str(adata.uns[key]["params"]["reference"])
     group_names = adata.uns[key]["names"].dtype.names if groups is None else groups
+    if isinstance(group_names, str):
+        group_names = [group_names]
     # one panel for each group
     # set up the figure
     n_panels_x = min(n_panels_per_row, len(group_names))
@@ -544,6 +546,8 @@ def _rank_genes_groups_plot(  # noqa: PLR0912, PLR0913, PLR0915
     if groupby is None:
         groupby = str(adata.uns[key]["params"]["groupby"])
     group_names = adata.uns[key]["names"].dtype.names if groups is None else groups
+    if isinstance(group_names, str):
+        group_names = [group_names]
 
     if var_names is not None:
         if isinstance(var_names, Mapping):

tests/plotting/legacy/test_plotting.py

@@ -926,6 +926,29 @@ def test_rank_genes_group_axes(plot_cmp):
     plt.close()
 
 
+@pytest.mark.parametrize(
+    ("plot_func", "kwargs"),
+    [
+        pytest.param(sc.pl.rank_genes_groups, {}, id="rank_genes_groups"),
+        pytest.param(
+            sc.pl.rank_genes_groups_dotplot,
+            {"dendrogram": False},
+            id="shared-rank-genes-groups-plots",
+        ),
+    ],
+)
+def test_rank_genes_groups_single_group_string(
+    plot_func: Callable[..., Any], kwargs: dict[str, Any]
+) -> None:
+    adata = pbmc68k_reduced()
+    sc.tl.rank_genes_groups(adata, "bulk_labels")
+
+    result = plot_func(adata, groups="Dendritic", n_genes=2, show=False, **kwargs)
+
+    assert result is not None
+    plt.close("all")
+
+
 @pytest.fixture(scope="session")
 def gene_symbols_adatas_session() -> tuple[AnnData, AnnData]:
     """Create two anndata objects which are equivalent except for var_names.