diff --git a/src/rtichoke/calibration/__init__.py b/src/rtichoke/calibration/__init__.py
index 190e74e9..b04d51eb 100644
--- a/src/rtichoke/calibration/__init__.py
+++ b/src/rtichoke/calibration/__init__.py
@@ -2,6 +2,30 @@
 Subpackage for Calibration
 """
 
-from .calibration import create_calibration_curve, create_calibration_curve_times
+from . import calibration as _calibration
+from ._interactive_aspect import enforce_square_calibration_panel
+
+_original_create_calibration_curve = _calibration.create_calibration_curve
+_original_create_calibration_curve_times = _calibration.create_calibration_curve_times
+
+
+def create_calibration_curve(*args, **kwargs):
+    """Create an interactive calibration plot with a square main panel."""
+    return enforce_square_calibration_panel(
+        _original_create_calibration_curve(*args, **kwargs)
+    )
+
+
+def create_calibration_curve_times(*args, **kwargs):
+    """Create an interactive time-dependent calibration plot with a square main panel."""
+    return enforce_square_calibration_panel(
+        _original_create_calibration_curve_times(*args, **kwargs)
+    )
+
+
+# Keep direct imports from rtichoke.calibration.calibration aligned with the
+# public package entry points.
+_calibration.create_calibration_curve = create_calibration_curve
+_calibration.create_calibration_curve_times = create_calibration_curve_times
 
 __all__ = ["create_calibration_curve", "create_calibration_curve_times"]
diff --git a/src/rtichoke/calibration/_interactive_aspect.py b/src/rtichoke/calibration/_interactive_aspect.py
new file mode 100644
index 00000000..de8c44ef
--- /dev/null
+++ b/src/rtichoke/calibration/_interactive_aspect.py
@@ -0,0 +1,27 @@
+"""Shared layout constraints for interactive calibration plots."""
+
+from typing import Any
+
+from plotly.graph_objs._figure import Figure
+
+
+def enforce_square_calibration_panel(fig: Figure) -> Figure:
+    """Keep the upper calibration panel on a 1:1 predicted/observed scale.
+
+    Calibration figures include a histogram in a separate lower subplot.  The
+    aspect-ratio constraint therefore belongs only to the upper calibration
+    panel, not to the full Plotly widget or to the histogram.
+    """
+    fig.update_yaxes(
+        scaleanchor="x",
+        scaleratio=1,
+        constrain="domain",
+        row=1,
+        col=1,
+    )
+    return fig
+
+
+def shared_calibration_axis_layout(axis_range: list[float]) -> dict[str, Any]:
+    """Return the common zoom settings used by calibration x and y axes."""
+    return {"range": axis_range, "fixedrange": False}
diff --git a/tests/test_calibration_interactive_aspect.py b/tests/test_calibration_interactive_aspect.py
new file mode 100644
index 00000000..30db43c6
--- /dev/null
+++ b/tests/test_calibration_interactive_aspect.py
@@ -0,0 +1,45 @@
+import numpy as np
+
+from rtichoke.calibration import create_calibration_curve, create_calibration_curve_times
+
+
+def _assert_square_main_panel(fig):
+    assert fig.layout.yaxis.scaleanchor == "x"
+    assert fig.layout.yaxis.scaleratio == 1
+    assert fig.layout.yaxis.constrain == "domain"
+    # The histogram has its own y axis and must remain unconstrained.
+    assert fig.layout.yaxis2.scaleanchor is None
+
+
+def test_interactive_calibration_main_panel_is_square():
+    probs = {"model": np.linspace(0.05, 0.95, 20)}
+    reals = np.array([0, 1] * 10)
+
+    for calibration_type in ("discrete", "smooth"):
+        fig = create_calibration_curve(
+            probs, reals, calibration_type=calibration_type
+        )
+        _assert_square_main_panel(fig)
+        assert list(fig.layout.xaxis.range) == list(fig.layout.yaxis.range)
+
+
+def test_interactive_calibration_times_main_panel_is_square():
+    probs = {"model": np.linspace(0.05, 0.95, 20)}
+    reals = np.array([0, 1] * 10)
+    times = np.arange(1.0, 21.0)
+
+    fig = create_calibration_curve_times(
+        probs,
+        reals,
+        times,
+        fixed_time_horizons=[10.0, 15.0],
+        heuristics_sets=[
+            {
+                "censoring_heuristic": "excluded",
+                "competing_heuristic": "excluded",
+            }
+        ],
+    )
+
+    _assert_square_main_panel(fig)
+    assert list(fig.layout.xaxis.range) == list(fig.layout.yaxis.range)