Add framework for un-converting units

dstansby · dstansby · commit 39ba96b33a03 · 2019-09-27T12:38:36.000+01:00
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
@@ -1576,6 +1576,16 @@ def convert_units(self, x):
                                          f'units: {x!r}') from e
         return ret
 
+    @property
+    def _can_unconvert_units(self):
+        if self.converter is not None:
+            if hasattr(self.converter, 'un_convert'):
+                return True
+        return False
+
+    def unconvert_units(self, x):
+        return self.converter.un_convert(x, self.units, self)
+
     def set_units(self, u):
         """
         Set the units for axis.
diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
@@ -1873,15 +1873,13 @@ def weeks(w):
     return w * DAYS_PER_WEEK
 
 
-class DateConverter(units.ConversionInterface):
+class BaseDateConverter(units.ConversionInterface):
     """
-    Converter for datetime.date and datetime.datetime data,
-    or for date/time data represented as it would be converted
-    by :func:`date2num`.
+    A base converter for datetime.date and datetime.datetime data, or for
+    date/time data represented as it would be converted by :func:`date2num`.
 
     The 'unit' tag for such data is None or a tzinfo instance.
     """
-
     @staticmethod
     def axisinfo(unit, axis):
         """
@@ -1930,6 +1928,24 @@ def default_units(x, axis):
         return None
 
 
+class DateConverter(BaseDateConverter):
+    @staticmethod
+    def un_convert(value, unit, axis):
+        return num2date(value)
+
+
+class Datetime64Converter(BaseDateConverter):
+    @staticmethod
+    def un_convert(value, unit, axis):
+        return np.datetime64(num2date(value).replace(tzinfo=None))
+
+
+class DatetimeConverter(BaseDateConverter):
+    @staticmethod
+    def un_convert(value, unit, axis):
+        return num2date(value)
+
+
 class ConciseDateConverter(DateConverter):
     """
     Converter for datetime.date and datetime.datetime data,
@@ -1968,6 +1984,6 @@ def axisinfo(self, unit, axis):
                               default_limits=(datemin, datemax))
 
 
-units.registry[np.datetime64] = DateConverter()
+units.registry[np.datetime64] = Datetime64Converter()
 units.registry[datetime.date] = DateConverter()
-units.registry[datetime.datetime] = DateConverter()
+units.registry[datetime.datetime] = DatetimeConverter()
diff --git a/lib/matplotlib/units.py b/lib/matplotlib/units.py
@@ -137,6 +137,14 @@ def convert(obj, unit, axis):
         """
         return obj
 
+    @staticmethod
+    def un_convert(data, unit, axis):
+        """
+        Convert data that has already been converted back to its original
+        value.
+        """
+        return data
+
     @staticmethod
     def is_numlike(x):
         """
