Make stem formatting parameters keyword only.

timhoffm · timhoffm · commit e68a2bf0428e · 2021-09-19T15:21:25.000+02:00
This will allow to simplify the implementation because, currently,
`stem(*args, linefmt=None, ...)` still tries to resolve excess
positionally passed args, to *linefmt* and following parameters, which
is quite a bit of logic.

OTOH, since we have 3 formats, passing them positionally is already
difficult from a usability/readability perspective, because they can
easily be mixed up.
diff --git a/doc/api/next_api_changes/deprecations/21126-TH.rst.py b/doc/api/next_api_changes/deprecations/21126-TH.rst.py
@@ -0,0 +1,2 @@
+Passing formatting parameters positionally to ``stem()`` is deprecated
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -2773,6 +2773,7 @@ def broken_barh(self, xranges, yrange, **kwargs):
 
         return col
 
+    @_api.make_keyword_only("3.5", 'linefmt')
     @_preprocess_data()
     def stem(self, *args, linefmt=None, markerfmt=None, basefmt=None, bottom=0,
              label=None, use_line_collection=True, orientation='vertical'):

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	+Passing formatting parameters positionally to ``stem()`` is deprecated
	`2`	`+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`