Fixes custom scale example, see matplotlib#5522

Added the property has_inverse = True to the custom scale example
so that if axhline is later called on this axis an error is not
produced.

Author: jeffrey-hokanson

examples/api/custom_scale_example.py

@@ -103,7 +103,7 @@ class MercatorLatitudeTransform(mtransforms.Transform):
         input_dims = 1
         output_dims = 1
         is_separable = True
-
+        has_inverse = True
         def __init__(self, thresh):
             mtransforms.Transform.__init__(self)
             self.thresh = thresh
@@ -138,6 +138,7 @@ class InvertedMercatorLatitudeTransform(mtransforms.Transform):
         input_dims = 1
         output_dims = 1
         is_separable = True
+        has_inverse = True
 
         def __init__(self, thresh):
             mtransforms.Transform.__init__(self)