Simplify color/marker disambiguation logic in _process_plot_format.

There's a bit of code in the parsing of plot() format shorthands to
disambiguate the "1" marker and the "1.0" grayscale color string.  It's
actually easier to just explicitly check for the bad strings.

This also makes it clearer that we *could* have supported "0" as
unambiguous color string (white) as there's no "0" marker (there's a
0 (int) marker, which is not the same...), but let's not bother with
changing any semantics.

Author: anntzer

lib/matplotlib/axes/_base.py

@@ -143,24 +143,16 @@ def _process_plot_format(fmt, *, ambiguous_fmt_datakey=False):
     marker = None
     color = None
 
-    # Is fmt just a colorspec?
-    try:
-        color = mcolors.to_rgba(fmt)
-
-        # We need to differentiate grayscale '1.0' from tri_down marker '1'
+    # First check whether fmt is just a colorspec, but specifically exclude the
+    # grayscale string "1" (not "1.0"), which is interpreted as the tri_down
+    # marker "1".  The grayscale string "0" could be unambiguously understood
+    # as a color (black) but also excluded for consistency.
+    if fmt not in ["0", "1"]:
         try:
-            fmtint = str(int(fmt))
+            color = mcolors.to_rgba(fmt)
+            return linestyle, marker, color
         except ValueError:
-            return linestyle, marker, color  # Yes
-        else:
-            if fmt != fmtint:
-                # user definitely doesn't want tri_down marker
-                return linestyle, marker, color  # Yes
-            else:
-                # ignore converted color
-                color = None
-    except ValueError:
-        pass  # No, not just a color.
+            pass
 
     errfmt = ("{!r} is neither a data key nor a valid format string ({})"
               if ambiguous_fmt_datakey else