plotly · Erikp15 · Jun 27, 2025 · Jun 27, 2025 · Jun 27, 2025 · Jun 27, 2025
diff --git a/plotly/basedatatypes.py b/plotly/basedatatypes.py
@@ -4093,6 +4093,8 @@ def _process_multiple_axis_spanning_shapes(
         augmented_annotation = shapeannotation.axis_spanning_shape_annotation(
             annotation, shape_type, shape_args, annotation_kwargs
         )
+        if exclude_empty_subplots and len(self.data) == 0:
+            exclude_empty_subplots = False
         self.add_shape(
             row=row,
             col=col,

diff --git a/tests/test_core/test_shapes/test_add_hline_empty_subplots.py b/tests/test_core/test_shapes/test_add_hline_empty_subplots.py
@@ -0,0 +1,31 @@
+import pytest
+import plotly.graph_objects as go
+from plotly.subplots import make_subplots
+
+
+def test_add_hline_on_empty_subplots_creates_shape_default():
+    fig = make_subplots(rows=1, cols=1)
+    fig.add_hline(y=0.25)
+    shapes = fig.layout.shapes
+    assert len(shapes) == 1, "Expected one shape for the horizontal line"
+    shape = shapes[0]
+    assert shape.type == "line"
+    assert shape.y0 == 0.25 and shape.y1 == 0.25
+    # xref and yref should be set
+    assert getattr(shape, "xref", None) is not None
+    assert getattr(shape, "yref", None) is not None
+
+
+@pytest.mark.parametrize("row, col", [(None, None), (1, 1)])
+def test_add_hline_with_explicit_row_col_on_empty_subplots(row, col):
+    fig = make_subplots(rows=1, cols=1)
+    # explicit row/col
+    fig.add_hline(y=0.5, row=row, col=col)
+    shapes = fig.layout.shapes
+    assert len(shapes) == 1, f"Expected one shape when row={row}, col={col}"
+    shape = shapes[0]
+    assert shape.y0 == 0.5 and shape.y1 == 0.5
+    assert shape.type == "line"
+    # ensure references default
+    assert shape.xref is not None
+    assert shape.yref is not None
diff --git a/tests/test_regression/test_hline_subplots_bug.py b/tests/test_regression/test_hline_subplots_bug.py
@@ -0,0 +1,61 @@
+import pytest
+import plotly.graph_objects as go
+from plotly.subplots import make_subplots
+
+
+def _apply_line(fig, orientation):
+    if orientation == "h":
+        fig.add_hline(y=0.5)
+    elif orientation == "v":
+        fig.add_vline(x=0.3)
+    else:
+        raise ValueError("orientation must be 'h' or 'v'")
+
+
+@pytest.mark.parametrize(
+    "orientation,kwargs",
+    [
+        (
+            "h",
+            dict(
+                line_coord_key="y0", coord=0.5, span_keys=("x0", "x1"), span_vals=(0, 1)
+            ),
+        ),
+        (
+            "v",
+            dict(
+                line_coord_key="x0", coord=0.3, span_keys=("y0", "y1"), span_vals=(0, 1)
+            ),
+        ),
+    ],
+)
+@pytest.mark.parametrize(
+    "constructor",
+    [
+        pytest.param(lambda: go.Figure(), id="plain-figure"),
+        pytest.param(lambda: make_subplots(rows=1, cols=1), id="make_subplots"),
+    ],
+)
+def test_add_line_presence(orientation, kwargs, constructor):
+    """Both add_hline and add_vline must create a shape, even on empty subplots."""
+    fig = constructor()
+
+    assert len(fig.data) == 0
+    assert len(fig.layout.shapes) == 0
+
+    _apply_line(fig, orientation)
+
+    # exactly one shape expected regardless of constructor and orientation
+    assert len(fig.layout.shapes) == 1, (
+        f"add_{orientation}line must create a shape on an empty figure; "
+        "currently fails for make_subplots."
+    )
+
+    shape = fig.layout.shapes[0]
+    # validate coordinate of line
+    assert pytest.approx(shape[kwargs["line_coord_key"]]) == kwargs["coord"]
+    # validate full-span of the other axis
+    span_key0, span_key1 = kwargs["span_keys"]
+    expected0, expected1 = kwargs["span_vals"]
+    assert shape[span_key0] == expected0
+    assert shape[span_key1] == expected1