BUG fixed by @anntzer: correct tick labels with subsampled FixedLocator

efiring · efiring · commit dd8e4b53c55a · 2020-04-28T15:25:57.000-10:00
When explicitly setting the tick locations and then the corresponding
tick labels, a FixedLocator is used for the locations.  It has an nbins
argument that can trigger subsampling.  Previously, tick labels were
made by FixedFormatter and based on position, not value, so subsampling
caused a mismatch, yielding incorrect tick labels.  This is changed to
use FuncFormatter with a function to look up the label by value,
ignoring the position.
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
@@ -1626,11 +1626,18 @@ def set_ticklabels(self, ticklabels, *, minor=False, **kwargs):
         """
         ticklabels = [t.get_text() if hasattr(t, 'get_text') else t
                       for t in ticklabels]
+        locator = (self.get_minor_locator() if minor
+                   else self.get_major_locator())
+        if isinstance(locator, mticker.FixedLocator):
+            formatter = mticker.FuncFormatter(
+                lambda x, pos: ticklabels[[*locator.locs].index(x)])
+        else:
+            formatter = mticker.FixedFormatter(ticklabels)
         if minor:
-            self.set_minor_formatter(mticker.FixedFormatter(ticklabels))
+            self.set_minor_formatter(formatter)
             ticks = self.get_minor_ticks()
         else:
-            self.set_major_formatter(mticker.FixedFormatter(ticklabels))
+            self.set_major_formatter(formatter)
             ticks = self.get_major_ticks()
         ret = []
         for tick_label, tick in zip(ticklabels, ticks):
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -381,8 +381,10 @@ class FuncFormatter(Formatter):
     position ``pos``), and return a string containing the corresponding
     tick label.
     """
+
     def __init__(self, func):
         self.func = func
+        self.offset_string = ""
 
     def __call__(self, x, pos=None):
         """
@@ -392,6 +394,12 @@ def __call__(self, x, pos=None):
         """
         return self.func(x, pos)
 
+    def get_offset(self):
+        return self.offset_string
+
+    def set_offset_string(self, ofs):
+        self.offset_string = ofs
+
 
 class FormatStrFormatter(Formatter):
     """
