matplotlib · jklymak · Sep 10, 2021 · Aug 30, 2021
diff --git a/lib/matplotlib/artist.py b/lib/matplotlib/artist.py
@@ -12,6 +12,7 @@
 
 import matplotlib as mpl
 from . import _api, cbook
+from .cm import ScalarMappable
 from .path import Path
 from .transforms import (Bbox, IdentityTransform, Transform, TransformedBbox,
                          TransformedPatchPath, TransformedPath)
@@ -1258,17 +1259,18 @@ def format_cursor_data(self, data):
         --------
         get_cursor_data
         """
-        if np.ndim(data) == 0 and getattr(self, "colorbar", None):
+        if np.ndim(data) == 0 and isinstance(self, ScalarMappable):
             # This block logically belongs to ScalarMappable, but can't be
             # implemented in it because most ScalarMappable subclasses inherit
             # from Artist first and from ScalarMappable second, so
             # Artist.format_cursor_data would always have precedence over
             # ScalarMappable.format_cursor_data.
-            return (
-                "["
-                + cbook.strip_math(
-                    self.colorbar.formatter.format_data_short(data)).strip()
-                + "]")
+            n = self.cmap.N
+            # Midpoints of neighboring color intervals.
+            neighbors = self.norm.inverse(
+                (int(self.norm(data) * n) + np.array([0, 1])) / n)
+            delta = abs(neighbors - data).max()
+            return "[{:-#.{}g}]".format(data, cbook._g_sig_digits(data, delta))
         else:
             try:
                 data[0]

diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
@@ -12,6 +12,7 @@
 import functools
 import gzip
 import itertools
+import math
 import operator
 import os
 from pathlib import Path
@@ -2201,6 +2202,23 @@ def _format_approx(number, precision):
     return f'{number:.{precision}f}'.rstrip('0').rstrip('.') or '0'
 
 
+def _g_sig_digits(value, delta):
+    """
+    Return the number of significant digits to %g-format *value*, assuming that
+    it is known with an error of *delta*.
+    """
+    # If e.g. value = 45.67 and delta = 0.02, then we want to round to 2 digits
+    # after the decimal point (floor(log10(0.02)) = -2); 45.67 contributes 2
+    # digits before the decimal point (floor(log10(45.67)) + 1 = 2): the total
+    # is 4 significant digits.  A value of 0 contributes 1 "digit" before the
+    # decimal point.
+    # For inf or nan, the precision doesn't matter.
+    return max(
+        0,
+        (math.floor(math.log10(abs(value))) + 1 if value else 1)
+        - math.floor(math.log10(delta))) if math.isfinite(value) else 0
+
+
 def _unikey_or_keysym_to_mplkey(unikey, keysym):
     """
     Convert a Unicode key or X keysym to a Matplotlib key name.

diff --git a/lib/matplotlib/projections/polar.py b/lib/matplotlib/projections/polar.py
@@ -1399,16 +1399,10 @@ def format_coord(self, theta, r):
         # (as for linear axes), but for theta, use f-formatting as scientific
         # notation doesn't make sense and the trailing dot is ugly.
         def format_sig(value, delta, opt, fmt):
-            digits_post_decimal = math.floor(math.log10(delta))
-            digits_offset = (
-                # For "f", only count digits after decimal point.
-                0 if fmt == "f"
-                # For "g", offset by digits before the decimal point.
-                else math.floor(math.log10(abs(value))) + 1 if value
-                # For "g", 0 contributes 1 "digit" before the decimal point.
-                else 1)
-            fmt_prec = max(0, digits_offset - digits_post_decimal)
-            return f"{value:-{opt}.{fmt_prec}{fmt}}"
+            # For "f", only count digits after decimal point.
+            prec = (max(0, -math.floor(math.log10(delta))) if fmt == "f" else
+                    cbook._g_sig_digits(value, delta))
+            return f"{value:-{opt}.{prec}{fmt}}"
 
         return ('\N{GREEK SMALL LETTER THETA}={}\N{GREEK SMALL LETTER PI} '
                 '({}\N{DEGREE SIGN}), r={}').format(

diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -337,11 +337,11 @@ def test_cursor_data():
 
 
 @pytest.mark.parametrize(
-    "data, text_without_colorbar, text_with_colorbar", [
-        ([[10001, 10000]], "[1e+04]", "[10001]"),
-        ([[.123, .987]], "[0.123]", "[0.123]"),
+    "data, text", [
+        ([[10001, 10000]], "[10001.000]"),
+        ([[.123, .987]], "[0.123]"),
     ])
-def test_format_cursor_data(data, text_without_colorbar, text_with_colorbar):
+def test_format_cursor_data(data, text):
     from matplotlib.backend_bases import MouseEvent
 
     fig, ax = plt.subplots()
@@ -350,15 +350,7 @@ def test_format_cursor_data(data, text_without_colorbar, text_with_colorbar):
     xdisp, ydisp = ax.transData.transform([0, 0])
     event = MouseEvent('motion_notify_event', fig.canvas, xdisp, ydisp)
     assert im.get_cursor_data(event) == data[0][0]
-    assert im.format_cursor_data(im.get_cursor_data(event)) \
-        == text_without_colorbar
-
-    fig.colorbar(im)
-    fig.canvas.draw()  # This is necessary to set up the colorbar formatter.
-
-    assert im.get_cursor_data(event) == data[0][0]
-    assert im.format_cursor_data(im.get_cursor_data(event)) \
-        == text_with_colorbar
+    assert im.format_cursor_data(im.get_cursor_data(event)) == text
 
 
 @image_comparison(['image_clip'], style='mpl20')

diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -655,16 +655,7 @@ def format_data_short(self, value):
                 # Rough approximation: no more than 1e4 divisions.
                 a, b = self.axis.get_view_interval()
                 delta = (b - a) / 1e4
-            # If e.g. value = 45.67 and delta = 0.02, then we want to round to
-            # 2 digits after the decimal point (floor(log10(0.02)) = -2);
-            # 45.67 contributes 2 digits before the decimal point
-            # (floor(log10(45.67)) + 1 = 2): the total is 4 significant digits.
-            # A value of 0 contributes 1 "digit" before the decimal point.
-            sig_digits = max(
-                0,
-                (math.floor(math.log10(abs(value))) + 1 if value else 1)
-                - math.floor(math.log10(delta)))
-            fmt = f"%-#.{sig_digits}g"
+            fmt = "%-#.{}g".format(cbook._g_sig_digits(value, delta))
         return self._format_maybe_minus_and_locale(fmt, value)
 
     def format_data(self, value):