diff --git a/lisa/analysis/tasks.py b/lisa/analysis/tasks.py
index bb2630cf8530c7d5423f9cc40999e515120990a4..18006a5e4be83178d3f3a905000fb5db16c5e339 100644
--- a/lisa/analysis/tasks.py
+++ b/lisa/analysis/tasks.py
@@ -779,6 +779,8 @@ class TasksAnalysis(TraceAnalysisBase):
         """
 
         def plotter(axis, local_fig):
+            nonlocal active_value, sleep_value
+
             # Adapt the steps height to the existing limits. This allows
             # re-using an existing axis that already contains some data.
             min_lim, max_lim = axis.get_ylim()