Raise an error when a make_subplots span overlaps another subplot

CHANGELOG.md

@@ -24,6 +24,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Fix `mpl_to_plotly` conversion of matplotlib contour lines and line collections: close contour rings ending with `Z` codes, support dash styles, convert date x-axes, separate disjoint subpaths with `None` separators, hide line collection traces from the legend by default, and group consecutive same-style lines into single traces [[#5770](https://github.com/plotly/plotly.py/pull/5770)], with thanks to @robertoffmoura for the contribution!
 - Fix `plotly.io.write_images` ignoring `layout.width`/`layout.height` (and the template's) and exporting every figure at the default 700x500 size; it now uses the same width/height priority as `write_image` and `to_image` [[#5784](https://github.com/plotly/plotly.py/pull/5784)], with thanks to @breken-ai for the contribution!
 - Fix color utilities mutating the list of colors passed to them [[#5836](https://github.com/plotly/plotly.py/pull/5836)], with thanks to @Enesuygurs for the contribution!
+- Fix `make_subplots` silently overlapping subplots when a cell covered by a `colspan` or `rowspan` also holds a subplot in `specs`; a `ValueError` naming the cell is now raised instead [[#5858](https://github.com/plotly/plotly.py/pull/5858)], with thanks to @LukeTheoJohnson for the contribution!
 
 
 ## [7.1.0] - 2026-09-15

plotly/_subplots.py

@@ -681,6 +681,15 @@ def _check_hv_spacing(dimsize, spacing, name, dimvarname, dimname):
                     "Some 'rowspan' value is too large for this subplot grid."
                 )
 
+            for rr in range(r, r_spanned + 1):
+                for cc in range(c, c_spanned + 1):
+                    if (rr, cc) != (r, c) and specs[rr][cc] is not None:
+                        raise ValueError(
+                            f"The subplot at grid position ({r + 1}, {c + 1}) "
+                            f"spans the cell at ({rr + 1}, {cc + 1}), so "
+                            f"specs[{rr}][{cc}] must be None."
+                        )
+
             # Get x domain using grid and colspan
             x_s = grid[r][c][0] + spec["l"]
 

tests/test_core/test_subplots/test_make_subplots.py

@@ -81,6 +81,16 @@ def test_specs_rowspan_too_big(self):
         with self.assertRaises(Exception):
             tls.make_subplots(rows=3, specs=[[{}], [None], [{"rowspan": 2}]])
 
+    def test_specs_colspan_overlaps_subplot(self):
+        with pytest.raises(ValueError, match=r"specs\[0\]\[1\] must be None"):
+            subplots.make_subplots(cols=3, specs=[[{"colspan": 2}, {}, None]])
+
+    def test_specs_rowspan_overlaps_subplot(self):
+        with pytest.raises(ValueError, match=r"specs\[1\]\[0\] must be None"):
+            subplots.make_subplots(
+                rows=2, cols=2, specs=[[{"rowspan": 2}, {}], [{}, {}]]
+            )
+
     def test_insets_wrong_type(self):
         with self.assertRaises(Exception):
             tls.make_subplots(insets="not going to work")