Add formatter config option

pygal/config.py

@@ -154,6 +154,10 @@ class Config(object):
         False, bool, "Value", "Display values in human readable format",
         "(ie: 12.4M)")
 
+    formatter = Key(
+        None, callable, "Value",
+        "Function used to convert values into human readable format")
+
     logarithmic = Key(
         False, bool, "Value", "Display values in logarithmic scale")
 

pygal/graph/base.py

@@ -80,7 +80,12 @@ class BaseGraph(object):
     @property
     def _format(self):
         """Return the value formatter for this graph"""
-        return humanize if self.human_readable else str
+        if self.formatter is not None:
+            return self.formatter
+        elif self.human_readable:
+            return humanize
+        else:
+            return str
 
     def _compute(self):
         """Initial computations to draw the graph"""