Merge pull request matplotlib#21318 from jklymak/fix-improve-shared-aspect-error

FIX: better error message for shared axes and axis('equal')

Author: timhoffm

lib/matplotlib/axes/_base.py

@@ -1939,10 +1939,12 @@ def apply_aspect(self, position=None):
 
         shared_x = self in self._shared_axes["x"]
         shared_y = self in self._shared_axes["y"]
-        # Not sure whether we need this check:
+
         if shared_x and shared_y:
-            raise RuntimeError("adjustable='datalim' is not allowed when both "
-                               "axes are shared")
+            raise RuntimeError("set_aspect(..., adjustable='datalim') or "
+                               "axis('equal') are not allowed when both axes "
+                               "are shared.  Try set_aspect(..., "
+                               "adjustable='box').")
 
         # If y is shared, then we are only allowed to change x, etc.
         if shared_y:
@@ -2042,7 +2044,6 @@ def axis(self, *args, emit=True, **kwargs):
                 self.set_autoscale_on(True)
                 self.set_aspect('auto')
                 self.autoscale_view(tight=False)
-                # self.apply_aspect()
                 if s == 'equal':
                     self.set_aspect('equal', adjustable='datalim')
                 elif s == 'scaled':

lib/matplotlib/tests/test_axes.py

@@ -4954,6 +4954,13 @@ def test_shared_with_aspect_3():
             assert round(expected, 4) == round(ax.get_aspect(), 4)
 
 
+def test_shared_aspect_error():
+    fig, axes = plt.subplots(1, 2, sharex=True, sharey=True)
+    axes[0].axis("equal")
+    with pytest.raises(RuntimeError, match=r"set_aspect\(..., adjustable="):
+        fig.draw_without_rendering()
+
+
 @pytest.mark.parametrize('twin', ('x', 'y'))
 def test_twin_with_aspect(twin):
     fig, ax = plt.subplots()