fix: conditional matplotlib backend for interactive + headless use

notebooks/exp_calibration.ipynb

@@ -22,13 +22,22 @@
    "outputs": [],
    "source": [
     "import matplotlib\n",
-    "matplotlib.use(\"Agg\")\n",
+    "# Use non-interactive backend only when no display is available (e.g. jupyter execute)\n",
+    "import matplotlib as mpl\n",
+    "try:\n",
+    "    import matplotlib.pyplot as plt\n",
+    "    # Check if we're in a headless environment\n",
+    "    import os\n",
+    "    if not os.environ.get(\"DISPLAY\") and not os.environ.get(\"WAYLAND_DISPLAY\"):\n",
+    "        mpl.use(\"Agg\")\n",
+    "except ImportError:\n",
+    "    mpl.use(\"Agg\")\n",
     "    import matplotlib.pyplot as plt\n",
     "import numpy as np\n",
     "\n",
     "from exp_data.pipeline import process_spectrum, process_run_sequence\n",
     "from exp_data.root_io import write_spectrum, read_spectrum\n",
-    "from exp_data.livetime import LivetimeDetermination"
+    "from exp_data.livetime import LivetimeDetermination\n"
    ]
   },
   {