Cleanup converter docs and StrCategoryConverter behavior (matplotlib#29059)

- Use getter in docs
- Use a single instance of StrCategoryConverter for all types
  - avoids warning since some methods convert to `np.str_` and others are
    just `str`, which previously got separate instances
  - added test

Author: ksunden

galleries/users_explain/axes/axes_units.py

@@ -236,7 +236,7 @@
 x = np.arange(100)
 ax = axs[0]
 ax.plot(x, x)
-label = f'Converter: {ax.xaxis.converter}\n '
+label = f'Converter: {ax.xaxis.get_converter()}\n '
 label += f'Locator: {ax.xaxis.get_major_locator()}\n'
 label += f'Formatter: {ax.xaxis.get_major_formatter()}\n'
 ax.set_xlabel(label)
@@ -245,7 +245,7 @@
 time = np.arange('1980-01-01', '1980-06-25', dtype='datetime64[D]')
 x = np.arange(len(time))
 ax.plot(time, x)
-label = f'Converter: {ax.xaxis.converter}\n '
+label = f'Converter: {ax.xaxis.get_converter()}\n '
 label += f'Locator: {ax.xaxis.get_major_locator()}\n'
 label += f'Formatter: {ax.xaxis.get_major_formatter()}\n'
 ax.set_xlabel(label)
@@ -255,7 +255,7 @@
 names = list(data.keys())
 values = list(data.values())
 ax.plot(names, values)
-label = f'Converter: {ax.xaxis.converter}\n '
+label = f'Converter: {ax.xaxis.get_converter()}\n '
 label += f'Locator: {ax.xaxis.get_major_locator()}\n'
 label += f'Formatter: {ax.xaxis.get_major_formatter()}\n'
 ax.set_xlabel(label)

lib/matplotlib/category.py

@@ -227,7 +227,8 @@ def update(self, data):
 
 
 # Register the converter with Matplotlib's unit framework
-units.registry[str] = StrCategoryConverter()
-units.registry[np.str_] = StrCategoryConverter()
-units.registry[bytes] = StrCategoryConverter()
-units.registry[np.bytes_] = StrCategoryConverter()
+# Intentionally set to a single instance
+units.registry[str] = \
+    units.registry[np.str_] = \
+    units.registry[bytes] = \
+    units.registry[np.bytes_] = StrCategoryConverter()

lib/matplotlib/tests/test_category.py

@@ -246,6 +246,14 @@ def test_update_plot(self, plotter):
         axis_test(ax.xaxis, ['a', 'b', 'd', 'c'])
         axis_test(ax.yaxis, ['e', 'g', 'f', 'a', 'b', 'd'])
 
+    def test_update_plot_heterogenous_plotter(self):
+        ax = plt.figure().subplots()
+        ax.scatter(['a', 'b'], ['e', 'g'])
+        ax.plot(['a', 'b', 'd'], ['f', 'a', 'b'])
+        ax.bar(['b', 'c', 'd'], ['g', 'e', 'd'])
+        axis_test(ax.xaxis, ['a', 'b', 'd', 'c'])
+        axis_test(ax.yaxis, ['e', 'g', 'f', 'a', 'b', 'd'])
+
     failing_test_cases = [("mixed", ['A', 3.14]),
                           ("number integer", ['1', 1]),
                           ("string integer", ['42', 42]),