Display bad format string in error message.

I had something like `for i in range(n): plot(..., f"oC{i}")` which
works for n up to 10, but for greater values one gets "oC10" which is
not supported (which is reasonable, as some single-digit strings are
actually *marker* abbreviations); mentioning the full format string in
the error message ("Unrecognized character 0 in format string 'oC10'")
makes it clearer.

Author: anntzer

lib/matplotlib/axes/_base.py

@@ -197,7 +197,7 @@ def _process_plot_format(fmt):
             i += 2
         else:
             raise ValueError(
-                'Unrecognized character %c in format string' % c)
+                f'Unrecognized character {c} in format string {fmt!r}')
 
     if linestyle is None and marker is None:
         linestyle = mpl.rcParams['lines.linestyle']