API: resolve unset vmin / vmax in all ScalarMapple based methods

tacaswell · tacaswell · commit 6286e1d2b8c6 · 2020-07-08T17:51:37.000-04:00
closes #17703
diff --git a/doc/api/prev_api_changes/api_changes_3.3.0/behaviour.rst b/doc/api/prev_api_changes/api_changes_3.3.0/behaviour.rst
@@ -312,3 +312,30 @@ rcParam is True.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The *colors* parameter will now default to :rc:`lines.color`, while previously it defaulted to 'k'.
+
+Aggressively autoscale clim in ``ScalerMappable`` classes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+
+Previously some plotting methods would defer autoscaling until the
+first draw if only one of the *vmin* or *vmax* keyword arguments was
+passed (`.Axes.scatter`, `.Axes.hexbin`, `.Axes.imshow`,
+`.Axes.pcolorfast`) but would scale based on the passed data if
+neither was passed (independent if the  *norm* kwarg).
+
+Other  methods (`.Axes.pcolor`, `.Axes.pcolormesh`)
+always autoscaled base on the initial data.
+
+All of the plotting methods now resolve, based on the passed data, the
+the unset *vmin* or *vmax* at init time based on the data passed.
+
+If you were relying on exactly one of the *vmin* or *vmax* remaining
+unset between the time when the method is called and the first time
+the figure is rendered you get back the old behavior by manually setting
+the relevant limit back to `None` ::
+
+  cm_obj.norm.vmin = None
+  # or
+  cm_obj.norm.vmax = None
+
+which will be resolved at during the draw process.
diff --git a/lib/matplotlib/cm.py b/lib/matplotlib/cm.py
@@ -258,8 +258,10 @@ def _scale_norm(self, norm, vmin, vmax):
                             "simultaneously is deprecated since %(since)s and "
                             "will become an error %(removal)s. Please pass "
                             "vmin/vmax directly to the norm when creating it.")
-        else:
-            self.autoscale_None()
+
+        # always resolve the autoscaling so we have concrete limits
+        # rather than deferring to draw time.
+        self.autoscale_None()
 
     def to_rgba(self, x, alpha=None, bytes=False, norm=True):
         """
