Backport PR matplotlib#32078: DOC: Clarify backslash escaping for hatch patterns

Author: andrzejnovak

galleries/examples/shapes_and_collections/hatch_style_reference.py

@@ -35,10 +35,16 @@ def hatches_plot(ax, h):
 
 # %%
 # Hatching patterns can be repeated to increase the density.
+#
+# .. note::
+#    In regular (non-raw) Python strings, backslashes must be escaped:
+#    ``'\\\\'`` and ``r'\\'`` are both the two-character hatch ``\\``.
+#    Forgetting this silently halves the density — ``'\\\\\\'`` is only a
+#    triple hatch and thus sparser than ``'//////'``.
 
 fig, axs = plt.subplots(2, 5, layout='constrained', figsize=(6.4, 3.2))
 
-hatches = ['//', '\\\\', '||', '--', '++', 'xx', 'oo', 'OO', '..', '**']
+hatches = ['//', r'\\', '||', '--', '++', 'xx', 'oo', 'OO', '..', '**']
 
 for ax, h in zip(axs.flat, hatches):
     hatches_plot(ax, h)
@@ -48,7 +54,7 @@ def hatches_plot(ax, h):
 
 fig, axs = plt.subplots(2, 5, layout='constrained', figsize=(6.4, 3.2))
 
-hatches = ['/o', '\\|', '|*', '-\\', '+o', 'x*', 'o-', 'O|', 'O.', '*-']
+hatches = ['/o', r'\|', '|*', r'\-', '+o', 'x*', 'o-', 'O|', 'O.', '*-']
 
 for ax, h in zip(axs.flat, hatches):
     hatches_plot(ax, h)

lib/matplotlib/patches.py

@@ -637,6 +637,9 @@ def set_hatch(self, hatch):
         hatchings are done.  If same letter repeats, it increases the
         density of hatching of that pattern.
 
+        In regular (non-raw) Python strings, backslashes must be doubled:
+        ``'\\\\'`` and ``r'\\'`` are both a double back-diagonal hatch.
+
         Parameters
         ----------
         hatch : {'/', '\\', '|', '-', '+', 'x', 'o', 'O', '.', '*'}