Added fallback, api change note and test

Author: Impaler343

doc/api/next_api_changes/behavior/30923-PR.rst

@@ -0,0 +1,4 @@
+Invalid linewidths coerced to 0
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Negative or non-finite (NaN, infinity) linewidth inputs are now
+automatically coerced to 0.

lib/matplotlib/backend_bases.py

@@ -928,8 +928,11 @@ def set_joinstyle(self, js):
         self._joinstyle = JoinStyle(js)
 
     def set_linewidth(self, w):
-        """Set the linewidth in points."""
-        self._linewidth = float(w)
+        """
+        Set the linewidth in points.
+        Any infinite, NaN, or negative values will be set to 0.
+        """
+        self._linewidth = float(w) if np.isfinite(w) and w > 0 else 0.0
 
     def set_url(self, url):
         """Set the url for links in compatible backends."""

lib/matplotlib/collections.py

@@ -664,6 +664,7 @@ def set_linewidth(self, lw):
         # scale all of the dash patterns.
         self._linewidths, self._linestyles = self._bcast_lwls(
             self._us_lw, self._us_linestyles)
+        self._linewidths = np.nan_to_num(self._linewidths, nan=0, posinf=0).clip(0)
         self.stale = True
 
     def set_linestyle(self, ls):

lib/matplotlib/tests/test_collections.py

@@ -1538,3 +1538,10 @@ def mock_draw_path_collection(self, gc, master_transform, paths, all_transforms,
     ax.add_collection(coll)
 
     plt.draw()
+
+
+@pytest.mark.parametrize('lw', [np.nan, -5.0, np.inf])
+def test_invalid_linewidths(lw):
+    lc = LineCollection([[(0, 0), (1, 1)]])
+    lc.set_linewidth(lw)
+    assert lc.get_linewidth() == 0.0