matplotlib · ksunden · Apr 25, 2023 · May 2, 2023 · May 2, 2023
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
@@ -671,6 +671,8 @@ def __init__(self, axes, *, pickradius=15):
         self._major_tick_kw = dict()
         self._minor_tick_kw = dict()
 
+        self._axisinfo = None
+
         self.clear()
         self._autoscale_on = True
 
@@ -1674,6 +1676,7 @@ def _update_axisinfo(self):
             return
 
         info = self.converter.axisinfo(self.units, self)
+        self._axisinfo = info
 
         if info is None:
             return
@@ -1821,6 +1824,11 @@ def _set_formatter(self, formatter, level):
             _api.warn_external('FixedFormatter should only be used together '
                                'with FixedLocator')
 
+        assert isinstance(formatter, mticker.Formatter)
+
+        if hasattr(self, "converter") and self.converter is not None:
+            formatter.validate_converter(self.converter, self._axisinfo)
+
         if level == self.major:
             self.isDefault_majfmt = False
         else:

diff --git a/lib/matplotlib/category.py b/lib/matplotlib/category.py
@@ -164,6 +164,12 @@ def _text(value):
             value = str(value)
         return value
 
+    def validate_converter(self, converter, axisinfo):
+        if not isinstance(converter, StrCategoryConverter):
+            _api.warn_external(
+                "Expected a StrCategoryConverter for StrCategoryFormatter"
+            )
+
 
 class UnitData:
     def __init__(self, data=None):

diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
@@ -651,6 +651,11 @@ def __call__(self, x, pos=0):
     def set_tzinfo(self, tz):
         self.tz = _get_tzinfo(tz)
 
+    def validate_converter(self, converter, axisinfo):
+        if axisinfo.description != "days since matplotlib epoch":
+            print(converter, axisinfo.majfmt)
+            _api.warn_external("Converter may not be compatible with date formatting.")
+
 
 class ConciseDateFormatter(ticker.Formatter):
     """
@@ -872,6 +877,10 @@ def get_offset(self):
     def format_data_short(self, value):
         return num2date(value, tz=self._tz).strftime('%Y-%m-%d %H:%M:%S')
 
+    def validate_converter(self, converter, axisinfo):
+        if axisinfo.description != "days since matplotlib epoch":
+            _api.warn_external("Converter may not be compatible with date formatting.")
+
 
 class AutoDateFormatter(ticker.Formatter):
     """
@@ -992,6 +1001,9 @@ def __call__(self, x, pos=None):
 
         return result
 
+    def validate_converter(self, converter, axisinfo):
+        self._formatter.validate_converter(converter, axisinfo)
+
 
 class rrulewrapper:
     """
@@ -1804,7 +1816,8 @@ def axisinfo(self, unit, axis):
         datemax = datetime.date(1970, 1, 2)
 
         return units.AxisInfo(majloc=majloc, majfmt=majfmt, label='',
-                              default_limits=(datemin, datemax))
+                              default_limits=(datemin, datemax),
+                              description="days since matplotlib epoch")
 
     @staticmethod
     def convert(value, unit, axis):
@@ -1861,7 +1874,8 @@ def axisinfo(self, unit, axis):
         datemin = datetime.date(1970, 1, 1)
         datemax = datetime.date(1970, 1, 2)
         return units.AxisInfo(majloc=majloc, majfmt=majfmt, label='',
-                              default_limits=(datemin, datemax))
+                              default_limits=(datemin, datemax),
+                              description="days since matplotlib epoch")
 
 
 class _SwitchableDateConverter:

diff --git a/lib/matplotlib/testing/jpl_units/EpochConverter.py b/lib/matplotlib/testing/jpl_units/EpochConverter.py
@@ -21,7 +21,15 @@ def axisinfo(unit, axis):
         # docstring inherited
         majloc = date_ticker.AutoDateLocator()
         majfmt = date_ticker.AutoDateFormatter(majloc)
-        return units.AxisInfo(majloc=majloc, majfmt=majfmt, label=unit)
+
+        # There is actually an off by one error here, but allow DateFormatters
+        descr = "days since matplotlib epoch"
+        return units.AxisInfo(
+            majloc=majloc,
+            majfmt=majfmt,
+            label=unit,
+            description=descr,
+        )
 
     @staticmethod
     def float2epoch(value, unit):

diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -253,6 +253,11 @@ def _set_locator(self, locator):
         """Subclasses may want to override this to set a locator."""
         pass
 
+    def validate_converter(self, converter, axisinfo):
+        """Raise an exception if the converter is not valid for this formatter."""
+        # By default, accept any converter
+        pass
+
 
 class NullFormatter(Formatter):
     """Always return the empty string."""

diff --git a/lib/matplotlib/ticker.pyi b/lib/matplotlib/ticker.pyi
@@ -1,6 +1,7 @@
 from matplotlib.axis import Axis
-from matplotlib.transforms import Transform
 from matplotlib.projections.polar import _AxisWrapper
+from matplotlib.transforms import Transform
+from matplotlib.units import ConversionInterface, AxisInfo
 
 from collections.abc import Callable, Sequence
 from typing import Any, Literal
@@ -31,6 +32,7 @@ class Formatter(TickHelper):
     def set_locs(self, locs: list[float]) -> None: ...
     @staticmethod
     def fix_minus(s: str) -> str: ...
+    def validate_converter(self, converter: ConversionInterface, axisinfo: AxisInfo) -> None: ...
 
 class NullFormatter(Formatter): ...
 

diff --git a/lib/matplotlib/units.py b/lib/matplotlib/units.py
@@ -78,7 +78,7 @@ class AxisInfo:
     """
     def __init__(self, majloc=None, minloc=None,
                  majfmt=None, minfmt=None, label=None,
-                 default_limits=None):
+                 default_limits=None, description=None):
         """
         Parameters
         ----------
@@ -89,8 +89,10 @@ def __init__(self, majloc=None, minloc=None,
         label : str, optional
             The default axis label.
         default_limits : optional
-            The default min and max limits of the axis if no data has
-            been plotted.
+            The default min and max limits of the axis if no data has been plotted.
+        description: str, optional
+            A human readable description which may additionally be used to validate
+            converters.
 
         Notes
         -----
@@ -103,6 +105,7 @@ def __init__(self, majloc=None, minloc=None,
         self.minfmt = minfmt
         self.label = label
         self.default_limits = default_limits
+        self.description = description
 
 
 class ConversionInterface: