From 5ce468e9494971cada98885c188f1f5e0e2ad175 Mon Sep 17 00:00:00 2001
From: Casey Brooks <casey.brooks@agyn.io>
Date: Thu, 25 Dec 2025 13:18:48 +0000
Subject: [PATCH] fix: reuse categorical unitdata on shared axes (#62)

Refs: task-283
---
 lib/matplotlib/category.py        | 13 +++++++
 lib/matplotlib/tests/test_axes.py | 62 +++++++++++++++++++++++++++++++
 2 files changed, 75 insertions(+)

diff --git a/lib/matplotlib/category.py b/lib/matplotlib/category.py
index 4ac2379ea5f5..dedbbde9d24b 100644
--- a/lib/matplotlib/category.py
+++ b/lib/matplotlib/category.py
@@ -101,6 +101,19 @@ def default_units(data, axis):
             object storing string to integer mapping
         """
         # the conversion call stack is default_units -> axis_info -> convert
+        if axis.units is None:
+            axis_name = axis.axis_name
+            shared_axes = axis.axes._shared_axes[axis_name].get_siblings(
+                axis.axes)
+            for shared in shared_axes:
+                if shared is axis.axes:
+                    continue
+                sibling_axis = shared._axis_map[axis_name]
+                if isinstance(sibling_axis.units, UnitData):
+                    axis.units = sibling_axis.units
+                    axis._update_axisinfo()
+                    break
+
         if axis.units is None:
             axis.set_units(UnitData(data))
         else:
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
index 4f01752819f4..cbc860661c03 100644
--- a/lib/matplotlib/tests/test_axes.py
+++ b/lib/matplotlib/tests/test_axes.py
@@ -23,6 +23,7 @@
 import matplotlib.dates as mdates
 from matplotlib.figure import Figure
 from matplotlib.axes import Axes
+from matplotlib.category import UnitData
 import matplotlib.font_manager as mfont_manager
 import matplotlib.markers as mmarkers
 import matplotlib.patches as mpatches
@@ -3036,6 +3037,67 @@ def layers(n, m):
     axs[1, 1].stackplot(range(100), d.T, baseline='weighted_wiggle')
 
 
+def _stackplot_sample_data():
+    x = ["16 May", "17 May"]
+    y1 = np.array([1, 2])
+    y2 = np.array([3, 1])
+    return x, y1, y2
+
+
+def test_stackplot_twinx_preserves_datalim():
+    fig, ax1 = plt.subplots()
+    x, y1, y2 = _stackplot_sample_data()
+    ax1.stackplot(x, y1, y2)
+    before = ax1.dataLim.intervaly
+
+    ax2 = ax1.twinx()
+    assert_allclose(ax1.dataLim.intervaly, before)
+
+    ax2.plot(x, [2, 4])
+    assert_allclose(ax1.dataLim.intervaly, before)
+
+
+def test_stackplot_twinx_reuses_unitdata():
+    fig, ax1 = plt.subplots()
+    x, y1, y2 = _stackplot_sample_data()
+    ax1.stackplot(x, y1, y2)
+
+    ax2 = ax1.twinx()
+    ax2.plot(x, [2, 4])
+
+    assert isinstance(ax1.xaxis.units, UnitData)
+    assert ax2.xaxis.units is ax1.xaxis.units
+
+
+def test_stackplot_twinx_numeric_x_units_none():
+    fig, ax1 = plt.subplots()
+    x = [0, 1]
+    y1 = np.array([1, 2])
+    y2 = np.array([3, 1])
+    ax1.stackplot(x, y1, y2)
+
+    ax2 = ax1.twinx()
+    ax2.plot(x, [2, 4])
+
+    assert ax1.xaxis.units is None
+    assert ax2.xaxis.units is None
+
+
+def test_stackplot_categorical_primary_axis_units():
+    fig, ax = plt.subplots()
+    x, y1, y2 = _stackplot_sample_data()
+    ax.stackplot(x, y1, y2)
+
+    assert isinstance(ax.xaxis.units, UnitData)
+
+    before = ax.dataLim.intervaly
+    ax.plot(x, [4, 5])
+    after = ax.dataLim.intervaly
+
+    assert after[0] <= before[0]
+    assert after[1] >= 5
+
+
 def _bxp_test_helper(
         stats_kwargs={}, transform_stats=lambda s: s, bxp_kwargs={}):
     np.random.seed(937)