[Bug]: LogFormatter minor ticks with minor_thresholds of (0,0) does not behave as documented

Bug summary

In investigating #25894 I discovered that despite being documented as:

To disable labeling of minor ticks when 'labelOnlyBase' is False, use minor_thresholds=(0, 0). This is the default for the "classic" style.

It actually only removes some minor tick labels (e.g. with base of 10: 11-14 will get labels, but 16-19 (15 is edge case, dependent on floating point rounding, 1.4, or 140 also similar))

Code for reproduction

import matplotlib.pyplot as plt
import matplotlib.ticker as mticker

fig, ax = plt.subplots()
ax.set_yscale("log")

formatter = mticker.LogFormatter(minor_thresholds=(0,0))
ax.yaxis.set_minor_formatter(formatter)

ax.set_ylim(10, 20)
plt.show()

Actual outcome

formatter(10) # '10'
formatter(11) # '11'
formatter(12) # '12'
formatter(13) # '13'
formatter(14) # '14'
formatter(15) # '15'
formatter(16) # ''

Expected outcome

formatter(10) # ''
formatter(11) # ''
formatter(12) # ''
formatter(13) # ''
formatter(14) # ''
formatter(15) # ''
formatter(16) # ''

Additional information

Only really affects plots that are zoomed in to much less than one decade, which arguably doesn't make sense to use a log scale, but still seems incongruous with the documented "this turns off minor ticks".

Has to do with when coeff rounds down to 1 in:

matplotlib/lib/matplotlib/ticker.py

Lines 980 to 988 in f051d94

    
           fx = math.log(x) / math.log(b) 
        
           is_x_decade = _is_close_to_int(fx) 
        
           exponent = round(fx) if is_x_decade else np.floor(fx) 
        
           coeff = round(b ** (fx - exponent)) 
        
           if self.labelOnlyBase and not is_x_decade: 
        
               return '' 
        
           if self._sublabels is not None and coeff not in self._sublabels: 
        
               return ''

I'm not quite sure what the proper solution is, perhaps making self._sublabels=set() instead?

Operating system

Linux

Matplotlib Version

3.8 (main)

Matplotlib Backend

No response

Python version

3.11

Jupyter version

No response

Installation

git checkout

I think the fix here is

diff --git i/lib/matplotlib/ticker.py w/lib/matplotlib/ticker.py
index 4c33e8c9af..5cb1ca6746 100644
--- i/lib/matplotlib/ticker.py
+++ w/lib/matplotlib/ticker.py
@@ -964,7 +964,7 @@ class LogFormatter(Formatter):
             # It's probably a colorbar with
             # a format kwarg setting a LogFormatter in the manner
             # that worked with 1.5.x, but that doesn't work now.
-            self._sublabels = {1}  # label powers of base
+            self._sublabels = np.array([1])  # label powers of base
             return
 
         b = self._base
@@ -995,17 +995,16 @@ class LogFormatter(Formatter):
 
         if numticks > self.minor_thresholds[0]:
             # Label only bases
-            self._sublabels = {1}
+            self._sublabels = np.array([1])
         elif numdec > self.minor_thresholds[1]:
             # Add labels between bases at log-spaced coefficients;
             # include base powers in case the locations include
             # "major" and "minor" points, as in colorbar.
-            c = np.geomspace(1, b, int(b)//2 + 1)
-            self._sublabels = set(np.round(c))
+            self._sublabels = np.geomspace(1, b, int(b)//2 + 1).round()
             # For base 10, this yields (1, 2, 3, 4, 6, 10).
         else:
             # Label all integer multiples of base**n.
-            self._sublabels = set(np.arange(1, b + 1))
+            self._sublabels = np.arange(1, b + 1)
 
     def _num_to_string(self, x, vmin, vmax):
         return self._pprint_val(x, vmax - vmin) if 1 <= x <= 10000 else f"{x:1.0e}"
@@ -1021,11 +1020,10 @@ class LogFormatter(Formatter):
         fx = math.log(x) / math.log(b)
         is_x_decade = _is_close_to_int(fx)
         exponent = round(fx) if is_x_decade else np.floor(fx)
-        coeff = round(b ** (fx - exponent))
 
         if self.labelOnlyBase and not is_x_decade:
             return ''
-        if self._sublabels is not None and coeff not in self._sublabels:
+        if self._sublabels is not None and not np.isin(x, self._sublabels * b**exponent):
             return ''
 
         vmin, vmax = self.axis.get_view_interval()

i.e. to detect whether if a minor tick is at a (sublabel * b**expoenent value) (e.g. 2*10^1 = 20), just compute all these values, rather than trying to round() the mantissa (this is wrong if we have subticks coming from an AutoLocator in the case of #12865.

Tests & PR left as an exercise.

Actually the fix above is likely wrong because it would break #12865 (we actually do want these intermediate ticks to be labeled). I'm not entirely sure what the clean fix would be, but the patch at #29698 (comment) still seems reasonable to me (even though it seems to be mostly working around the underlying issue).

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[Bug]: LogFormatter minor ticks with `minor_thresholds` of (0,0) does not behave as documented #25896

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	fx = math.log(x) / math.log(b)
	is_x_decade = _is_close_to_int(fx)
	exponent = round(fx) if is_x_decade else np.floor(fx)
	coeff = round(b ** (fx - exponent))

	if self.labelOnlyBase and not is_x_decade:
	return ''
	if self._sublabels is not None and coeff not in self._sublabels:
	return ''

[Bug]: LogFormatter minor ticks with minor_thresholds of (0,0) does not behave as documented #25896

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anntzer commented on Mar 6, 2025

anntzer commented on Mar 11, 2025

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[Bug]: LogFormatter minor ticks with `minor_thresholds` of (0,0) does not behave as documented #25896