ENH: Add warning for SymLogScale when values in linear scale

parthpankajtiwary · parthpankajtiwary · commit 15571fc7d9a2 · 2022-11-27T19:36:59.000+01:00
diff --git a/lib/matplotlib/scale.py b/lib/matplotlib/scale.py
@@ -389,6 +389,11 @@ def __init__(self, base, linthresh, linscale):
 
     def transform_non_affine(self, a):
         abs_a = np.abs(a)
+        if (abs_a < self.linthresh).all():
+            _api.warn_external(
+                "All values for SymLogScale are below linthresh, making "
+                "it effectively linear. You likely should lower the value "
+                "of linthresh. ")
         with np.errstate(divide="ignore", invalid="ignore"):
             out = np.sign(a) * self.linthresh * (
                 np.power(self.base,
diff --git a/lib/matplotlib/tests/test_scale.py b/lib/matplotlib/tests/test_scale.py
@@ -14,6 +14,8 @@
 import io
 import pytest
 
+import random
+
 
 @check_figures_equal()
 def test_log_scales(fig_test, fig_ref):
@@ -55,6 +57,25 @@ def test_symlog_mask_nan():
     assert type(out) == type(x)
 
 
+def test_symlog_linthresh():
+    fig, ax = plt.subplots()
+
+    n_samples = 100
+
+    upper_bound = 1.0
+
+    x = [random.uniform(0.0, upper_bound) for _ in range(n_samples)]
+    y = [random.uniform(0.0, upper_bound) for _ in range(n_samples)]
+
+    with pytest.warns(UserWarning) as record:
+        plt.plot(x, y, 'o')
+        ax.set_xscale('symlog')
+        ax.set_yscale('symlog')
+        plt.show()
+
+    assert len(record) == 1
+
+
 @image_comparison(['logit_scales.png'], remove_text=True)
 def test_logit_scales():
     fig, ax = plt.subplots()
