Prepare for making create_dummy_axis not necessary.

create_dummy_axis() sets up some internal _DummyAxis, which should not
really have been leaked to end users; instead, we should just always
attach a _DummyAxis to locators and formatters at init to start with.

_DummyAxis should actually be quite cheap to create, but it does create
two Bbox.unit() (dataLim and viewLim) which are *slightly* expensive to
create, can be placed by plain tuples, and do not correspond to any APIs
on "standard" Axises (there's Axes.dataLim/viewLim, not
Axis.dataLim/viewLim).  While the offending attributes are on a private
class, they are publically accessible e.g. via
`locator.create_dummy_axis(); locator.axis` so go through a deprecation
to be extra safe.

Author: anntzer

doc/api/next_api_changes/deprecations/22554-AL.rst

@@ -0,0 +1,4 @@
+``_DummyAxis.dataLim`` and ``_DummyAxis.viewLim``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... are deprecated.  Use ``get_data_interval()``, ``set_data_interval()``,
+``get_view_interval()``, and ``set_view_interval()`` instead.

lib/matplotlib/ticker.py

@@ -154,25 +154,32 @@
 class _DummyAxis:
     __name__ = "dummy"
 
+    # Once the deprecation elapses, replace dataLim and viewLim by plain
+    # _view_interval and _data_interval private tuples.
+    dataLim = _api.deprecate_privatize_attribute(
+        "3.6", alternative="get_data_interval() and set_data_interval()")
+    viewLim = _api.deprecate_privatize_attribute(
+        "3.6", alternative="get_view_interval() and set_view_interval()")
+
     def __init__(self, minpos=0):
-        self.dataLim = mtransforms.Bbox.unit()
-        self.viewLim = mtransforms.Bbox.unit()
+        self._dataLim = mtransforms.Bbox.unit()
+        self._viewLim = mtransforms.Bbox.unit()
         self._minpos = minpos
 
     def get_view_interval(self):
-        return self.viewLim.intervalx
+        return self._viewLim.intervalx
 
     def set_view_interval(self, vmin, vmax):
-        self.viewLim.intervalx = vmin, vmax
+        self._viewLim.intervalx = vmin, vmax
 
     def get_minpos(self):
         return self._minpos
 
     def get_data_interval(self):
-        return self.dataLim.intervalx
+        return self._dataLim.intervalx
 
     def set_data_interval(self, vmin, vmax):
-        self.dataLim.intervalx = vmin, vmax
+        self._dataLim.intervalx = vmin, vmax
 
     def get_tick_space(self):
         # Just use the long-standing default of nbins==9