Improve error messages for unit conversion

timhoffm · timhoffm · commit 387c34537b63 · 2018-12-17T16:44:24.000+01:00
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
@@ -1486,8 +1486,11 @@ def convert_units(self, x):
 
         if self.converter is None:
             return x
-
-        ret = self.converter.convert(x, self.units, self)
+        try:
+            ret = self.converter.convert(x, self.units, self)
+        except Exception as e:
+            raise munits.ConversionError('Failed to convert value(s) to axis '
+                                         'units: %r' % x) from e
         return ret
 
     def set_units(self, u):
diff --git a/lib/matplotlib/category.py b/lib/matplotlib/category.py
@@ -29,18 +29,23 @@ def convert(value, unit, axis):
         Parameters
         ----------
         value : string or iterable
-            value or list of values to be converted
-        unit : :class:`.UnitData`
-           object string unit information for value
-        axis : :class:`~matplotlib.Axis.axis`
-            axis on which the converted value is plotted
+            Value or list of values to be converted.
+        unit : `.UnitData`
+            An object mapping strings to integers.
+        axis : `~matplotlib.axis.Axis`
+            axis on which the converted value is plotted.
+
+            .. note:: *axis* is unused.
 
         Returns
         -------
-        mapped_ value : float or ndarray[float]
-
-        .. note:: axis is not used in this function
+        mapped_value : float or ndarray[float]
         """
+        if unit is None:
+            raise ValueError(
+                'Missing unit for StrCategoryConverter. This might be caused'
+                'by unintendedly mixing categorical and numeric data.')
+
         # dtype = object preserves numerical pass throughs
         values = np.atleast_1d(np.array(value, dtype=object))
 
diff --git a/lib/matplotlib/units.py b/lib/matplotlib/units.py
@@ -49,6 +49,10 @@ def default_units(x, axis):
 from matplotlib import cbook
 
 
+class ConversionError(ValueError):
+    pass
+
+
 class AxisInfo(object):
     """
     Information to support default axis labeling, tick labeling, and
