Deprecate old and little used formatters.

- The new ScalarFormatter replaced OldScalarFormatter in 3910bb9 (2005).
- IndexFormatter appeared in e66091d (2009) but was never used in the
  examples; IndexDateFormatter is unused in the docs since 4ea4b96
  (2009); both can be fairly easily implemented in terms of
  FuncFormatter.

I'm willing to back off the deprecations if there's demand to keep them,
though.

Author: anntzer

doc/api/next_api_changes/deprecations.rst

@@ -144,3 +144,12 @@ and containment checks) via `.Line2D.set_picker` is deprecated.  Use
 
 `.Line2D.set_picker` no longer sets the artist's custom-contain() check.  Use
 ``Line2D.set_contains`` instead.
+
+``OldScalarFormatter``, ``IndexFormatter`` and ``DateIndexFormatter``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+These formatters are deprecated.  Their functionality can be implemented using
+e.g. `.FuncFormatter`.
+
+``OldAutoLocator``
+~~~~~~~~~~~~~~~~~~
+This ticker is deprecated.

lib/matplotlib/dates.py

@@ -604,6 +604,7 @@ def set_tzinfo(self, tz):
         self.tz = tz
 
 
+@cbook.deprecated("3.3")
 class IndexDateFormatter(ticker.Formatter):
     """Use with `.IndexLocator` to cycle format strings by index."""
 

lib/matplotlib/tests/test_ticker.py

@@ -10,6 +10,7 @@
 import pytest
 
 import matplotlib
+from matplotlib import cbook
 import matplotlib.pyplot as plt
 import matplotlib.ticker as mticker
 
@@ -457,7 +458,8 @@ class TestIndexFormatter:
                                           (2, 'label2'),
                                           (2.5, '')])
     def test_formatting(self, x, label):
-        formatter = mticker.IndexFormatter(['label0', 'label1', 'label2'])
+        with cbook._suppress_matplotlib_deprecation_warning():
+            formatter = mticker.IndexFormatter(['label0', 'label1', 'label2'])
         assert formatter(x) == label
 
 

lib/matplotlib/ticker.py

@@ -293,6 +293,7 @@ def _set_locator(self, locator):
         pass
 
 
+@cbook.deprecated("3.3")
 class IndexFormatter(Formatter):
     """
     Format the position x to the nearest i-th label where ``i = int(x + 0.5)``.
@@ -322,13 +323,10 @@ def __call__(self, x, pos=None):
 
 
 class NullFormatter(Formatter):
-    """
-    Always return the empty string.
-    """
+    """Always return the empty string."""
+
     def __call__(self, x, pos=None):
-        """
-        Returns an empty string for all inputs.
-        """
+        # docstring inherited
         return ''
 
 
@@ -428,6 +426,7 @@ def __call__(self, x, pos=None):
         return self.fmt.format(x=x, pos=pos)
 
 
+@cbook.deprecated("3.3")
 class OldScalarFormatter(Formatter):
     """
     Tick location is a plain old number.
@@ -2867,6 +2866,7 @@ def tick_values(self, vmin, vmax):
                                   '%s type.' % type(self))
 
 
+@cbook.deprecated("3.3")
 class OldAutoLocator(Locator):
     """
     On autoscale this class picks the best MultipleLocator to set the