diff --git a/panoramix/viz.py b/panoramix/viz.py
index 4295a4a35..4b918d8ff 100644
--- a/panoramix/viz.py
+++ b/panoramix/viz.py
@@ -420,7 +420,7 @@ class NVD3TimeSeriesViz(NVD3Viz):
         ('show_brush', 'show_legend'),
         ('rich_tooltip', 'y_axis_zero'),
         ('y_log_scale', 'contribution'),
-        'y_axis_format'
+        ('y_axis_format', None)
     ]
 
     def get_df(self, query_obj=None):