fix(plotting): Use Agg backend only during testing with pytest (#415)

src/pymovements/plotting/traceplot.py

@@ -22,6 +22,8 @@
 """
 from __future__ import annotations
 
+import sys
+
 import matplotlib
 import matplotlib.pyplot as plt
 import numpy as np
@@ -30,8 +32,11 @@
 
 from pymovements.gaze.gaze_dataframe import GazeDataFrame
 
-
-matplotlib.use('Agg')
+# This is really a dirty workaround to use the Agg backend if runnning pytest.
+# This is needed as Windows workers on GitHub fail randomly with other backends.
+# Unfortunately the Agg module cannot show plots in jupyter notebooks.
+if 'pytest' in sys.modules:  # pragma: no cover
+    matplotlib.use('Agg')
 
 DEFAULT_SEGMENTDATA = {
     'red': [
@@ -79,7 +84,7 @@ def traceplot(
         gaze: GazeDataFrame,
         x: str,
         y: str,
-        cval: np.ndarray | None = None,
+        cval: np.ndarray | None = None,  # pragma: no cover
         cmap: colors.Colormap | None = None,
         cmap_norm: colors.Normalize | str | None = None,
         cmap_segmentdata: dict[str, list[list[float]]] | None = None,