diff --git a/plotter.py b/plotter.py
index bd79d89..4503634 100644
--- a/plotter.py
+++ b/plotter.py
@@ -72,8 +72,10 @@ class PlotRule(Rule):
         return self.process_fn(arg, **kwargs)
 
     def datafile(self, name, arg):
+        if arg is not None:
+            name = name + "_" + str(arg)
         return Datafile(
-            name=name + "_" + arg,
+            name=name,
             description=self.description + " ({})".format(arg),
         )