Fix legend of colour-mapped scatter plots.

This is a slight backport of ed6d92b,
namely ignoring the array update status when updating the scalar
mappable.

To not break the watchers though, the check must still be called to
update the status.

Fixes #19779.

lib/matplotlib/collections.py

@@ -906,8 +906,11 @@ def update_scalarmappable(self):
         """
         if not self._set_mappable_flags():
             return
+        # We don't use the result of this call, but update the status for
+        # anyone watching.
+        self._check_update("array")
         # Allow possibility to call 'self.set_array(None)'.
-        if self._check_update("array") and self._A is not None:
+        if self._A is not None:
             # QuadMesh can map 2d arrays (but pcolormesh supplies 1d array)
             if self._A.ndim > 1 and not isinstance(self, QuadMesh):
                 raise ValueError('Collections can only map rank 1 arrays')

lib/matplotlib/tests/test_axes.py

@@ -52,14 +52,16 @@ def test_get_labels():
 @check_figures_equal()
 def test_label_loc_vertical(fig_test, fig_ref):
     ax = fig_test.subplots()
-    sc = ax.scatter([1, 2], [1, 2], c=[1, 2])
+    sc = ax.scatter([1, 2], [1, 2], c=[1, 2], label='scatter')
+    ax.legend()
     ax.set_ylabel('Y Label', loc='top')
     ax.set_xlabel('X Label', loc='right')
     cbar = fig_test.colorbar(sc)
     cbar.set_label("Z Label", loc='top')
 
     ax = fig_ref.subplots()
-    sc = ax.scatter([1, 2], [1, 2], c=[1, 2])
+    sc = ax.scatter([1, 2], [1, 2], c=[1, 2], label='scatter')
+    ax.legend()
     ax.set_ylabel('Y Label', y=1, ha='right')
     ax.set_xlabel('X Label', x=1, ha='right')
     cbar = fig_ref.colorbar(sc)
@@ -69,14 +71,16 @@ def test_label_loc_vertical(fig_test, fig_ref):
 @check_figures_equal()
 def test_label_loc_horizontal(fig_test, fig_ref):
     ax = fig_test.subplots()
-    sc = ax.scatter([1, 2], [1, 2], c=[1, 2])
+    sc = ax.scatter([1, 2], [1, 2], c=[1, 2], label='scatter')
+    ax.legend()
     ax.set_ylabel('Y Label', loc='bottom')
     ax.set_xlabel('X Label', loc='left')
     cbar = fig_test.colorbar(sc, orientation='horizontal')
     cbar.set_label("Z Label", loc='left')
 
     ax = fig_ref.subplots()
-    sc = ax.scatter([1, 2], [1, 2], c=[1, 2])
+    sc = ax.scatter([1, 2], [1, 2], c=[1, 2], label='scatter')
+    ax.legend()
     ax.set_ylabel('Y Label', y=0, ha='left')
     ax.set_xlabel('X Label', x=0, ha='left')
     cbar = fig_ref.colorbar(sc, orientation='horizontal')
@@ -88,14 +92,16 @@ def test_label_loc_rc(fig_test, fig_ref):
     with matplotlib.rc_context({"xaxis.labellocation": "right",
                                 "yaxis.labellocation": "top"}):
         ax = fig_test.subplots()
-        sc = ax.scatter([1, 2], [1, 2], c=[1, 2])
+        sc = ax.scatter([1, 2], [1, 2], c=[1, 2], label='scatter')
+        ax.legend()
         ax.set_ylabel('Y Label')
         ax.set_xlabel('X Label')
         cbar = fig_test.colorbar(sc, orientation='horizontal')
         cbar.set_label("Z Label")
 
     ax = fig_ref.subplots()
-    sc = ax.scatter([1, 2], [1, 2], c=[1, 2])
+    sc = ax.scatter([1, 2], [1, 2], c=[1, 2], label='scatter')
+    ax.legend()
     ax.set_ylabel('Y Label', y=1, ha='right')
     ax.set_xlabel('X Label', x=1, ha='right')
     cbar = fig_ref.colorbar(sc, orientation='horizontal')