From d11022e421255b380c1cbee6f593a2c6fd813c70 Mon Sep 17 00:00:00 2001
From: Aitik Gupta <aitikgupta@gmail.com>
Date: Wed, 26 May 2021 01:04:14 +0530
Subject: [PATCH 1/2] Warn user when mathtext font is used for ticks

---
 lib/matplotlib/ticker.py | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
index 2473bee2b0a0..2831069ab222 100644
--- a/lib/matplotlib/ticker.py
+++ b/lib/matplotlib/ticker.py
@@ -475,6 +475,22 @@ def __init__(self, useOffset=None, useMathText=None, useLocale=None):
         self._usetex = mpl.rcParams['text.usetex']
         if useMathText is None:
             useMathText = mpl.rcParams['axes.formatter.use_mathtext']
+            if useMathText is False:
+                try:
+                    ufont = mpl.font_manager.findfont(
+                        mpl.font_manager.FontProperties(
+                            mpl.rcParams["font.family"]
+                        ),
+                        fallback_to_default=False,
+                    )
+                except ValueError:
+                    ufont = None
+
+                if ufont == str(cbook._get_data_path("fonts/ttf/cmr10.ttf")):
+                    _api.warn_external(
+                        "cmr10 font should ideally be used with "
+                        "mathtext, set axes.formatter.use_mathtext to True"
+                    )
         self.set_useMathText(useMathText)
         self.orderOfMagnitude = 0
         self.format = ''

From 7a8aa99188c675ba87d585f77d580edf29ba396a Mon Sep 17 00:00:00 2001
From: Aitik Gupta <aitikgupta@gmail.com>
Date: Wed, 26 May 2021 01:04:39 +0530
Subject: [PATCH 2/2] Add test to trigger warning

---
 lib/matplotlib/tests/test_ticker.py | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/lib/matplotlib/tests/test_ticker.py b/lib/matplotlib/tests/test_ticker.py
index 2a6719e2fd3c..56ca73905cc5 100644
--- a/lib/matplotlib/tests/test_ticker.py
+++ b/lib/matplotlib/tests/test_ticker.py
@@ -577,6 +577,18 @@ def test_cursor_dummy_axis(self, data, expected):
         fmt = sf.format_data_short
         assert fmt(data) == expected
 
+    def test_mathtext_ticks(self):
+        mpl.rcParams.update({
+            'font.family': 'serif',
+            'font.serif': 'cmr10',
+            'axes.formatter.use_mathtext': False
+        })
+
+        with pytest.warns(UserWarning, match='cmr10 font should ideally'):
+            fig, ax = plt.subplots()
+            ax.set_xticks([-1, 0, 1])
+            fig.canvas.draw()
+
 
 class FakeAxis:
     """Allow Formatter to be called without having a "full" plot set up."""