diff --git a/system/workflow.py b/system/workflow.py
index 6780224..b993a2d 100644
--- a/system/workflow.py
+++ b/system/workflow.py
@@ -177,8 +177,14 @@ class WorkflowView (gtk.VBox):
 
         args, varargs, varkw, defaults = inspect.getargspec(function.run)
 
-        # first argument is 'self' and we don't care about that...
-        args = args[1:]
+        # first argument is 'self' and second should be the selection
+        # and we don't care about those...
+        args.remove('self')
+        if "selection" in args:
+            pass_selection = True
+            args.remove('selection')
+        else:
+            pass_selection = False
 
         if varargs and len(parent_data) < len(args):
             logger.log('warning', "Function requires minimum %d datasets selected." % len(args))
@@ -188,14 +194,20 @@ class WorkflowView (gtk.VBox):
             logger.log('warning', "Function requires %d datasets, but only %d selected." % (len(args), len(parent_data)))
             return
 
-        # we allow functions requiring no data to be run even if
-        # something is selected
-
         if not args:
-            new_data = function.run()
+            # we allow functions requiring no data to be run even if a
+            # dataset is is selected
+            data = []
         else:
-            new_data = function.run(*parent_data)
-
+            data = parent_data
+            
+        if pass_selection:
+            # if the function has a 'selection' argument, we pass in
+            # the selection
+            new_data = function.run(selection=project.get_selection(), *data)
+        else:
+            new_data = function.run(*data)
+            
         if new_data != None:
             project.add_data(parent_data, new_data, function.name)