Make value formatting fully configurable (through a lambda).

pygal/config.py

@@ -154,6 +154,10 @@ class Config(object):
         False, bool, "Value", "Display values in human readable format",
         "(ie: 12.4M)")
 
+    value_format = Key(
+        None, type(lambda: 1), "Value",
+        "A function to convert numeric value to strings")
+
     logarithmic = Key(
         False, bool, "Value", "Display values in logarithmic scale")
 

pygal/graph/base.py

@@ -80,7 +80,7 @@ class BaseGraph(object):
     @property
     def _format(self):
         """Return the value formatter for this graph"""
-        return humanize if self.human_readable else str
+        return self.value_format or (humanize if self.human_readable else str)
 
     def _compute(self):
         """Initial computations to draw the graph"""