fix for #375 (#377)

* fix for #375

* comment

src/akkudoktoreos/utils/visualize.py

@@ -5,6 +5,7 @@
 from collections.abc import Sequence
 from typing import Callable, Optional, Union
 
+import matplotlib
 import matplotlib.dates as mdates
 import matplotlib.pyplot as plt
 import numpy as np
@@ -17,6 +18,9 @@
 from akkudoktoreos.utils.datetimeutil import to_datetime
 
 logger = get_logger(__name__)
+matplotlib.use(
+    "Agg"
+)  # non-interactive backend that can only write to files, backend needed to stay in main thread.
 
 
 class VisualizationReport(ConfigMixin):