diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
index d6e4f50a2d7e..b93f8aec3142 100644
--- a/lib/matplotlib/axes/_axes.py
+++ b/lib/matplotlib/axes/_axes.py
@@ -64,11 +64,11 @@ def _plot_args_replacer(args, data):
         except ValueError:
             pass
         else:
-            warnings.warn(
+            cbook._warn_external(
                 "Second argument {!r} is ambiguous: could be a color spec but "
                 "is in data; using as data.  Either rename the entry in data "
                 "or use three arguments to plot.".format(args[1]),
-                RuntimeWarning, stacklevel=3)
+                RuntimeWarning)
         return ["x", "y"]
     elif len(args) == 3:
         return ["x", "y", "c"]
diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
index cd2177e75f6b..395ade0f649f 100644
--- a/lib/matplotlib/cbook/__init__.py
+++ b/lib/matplotlib/cbook/__init__.py
@@ -14,7 +14,7 @@
 import glob
 import gzip
 import io
-from itertools import repeat
+import itertools
 import locale
 import numbers
 import operator
@@ -1254,7 +1254,7 @@ def _compute_conf_interval(data, med, iqr, bootstrap):
 
     ncols = len(X)
     if labels is None:
-        labels = repeat(None)
+        labels = itertools.repeat(None)
     elif len(labels) != ncols:
         raise ValueError("Dimensions of labels and X must be compatible")
 
@@ -2032,3 +2032,21 @@ def _setattr_cm(obj, **kwargs):
                 delattr(obj, attr)
             else:
                 setattr(obj, attr, orig)
+
+
+def _warn_external(message, category=None):
+    """
+    `warnings.warn` wrapper that sets *stacklevel* to "outside Matplotlib".
+
+    The original emitter of the warning can be obtained by patching this
+    function back to `warnings.warn`, i.e. ``cbook._warn_external =
+    warnings.warn`` (or ``functools.partial(warnings.warn, stacklevel=2)``,
+    etc.).
+    """
+    frame = sys._getframe()
+    for stacklevel in itertools.count(1):
+        if not re.match(r"\A(matplotlib|mpl_toolkits)(\Z|\.)",
+                        frame.f_globals["__name__"]):
+            break
+        frame = frame.f_back
+    warnings.warn(message, category, stacklevel)