Allow use of noninteractive backend for plot_detector_models. Add test

newsfragments/XXX.bugfix

@@ -0,0 +1 @@
+``dxtbx.plot_detector_models``: use noninteractive matpotlib backend if using the pdf_file option

src/dxtbx/command_line/plot_detector_models.py

@@ -6,7 +6,6 @@
 import sys
 from collections.abc import Sequence
 
-import matplotlib.pyplot as plt
 import numpy as np
 from matplotlib.backends.backend_pdf import PdfPages
 from matplotlib.patches import FancyArrowPatch
@@ -170,6 +169,12 @@ def run(args=None):
             except Exception:
                 raise Sorry("Unrecognized argument %s" % arg)
     params = phil_scope.fetch(sources=user_phil).extract()
+    if params.pdf_file:
+        import matplotlib
+
+        matplotlib.use("Agg")
+
+    import matplotlib.pyplot as plt
 
     fig = plt.figure()
     colormap = plt.cm.gist_ncar

tests/command_line/test_plot_detector_models.py

@@ -0,0 +1,17 @@
+from __future__ import annotations
+
+import os.path
+import shutil
+import subprocess
+
+
+def test_plot_models_noninteractive_to_pdf(dials_data, tmp_path):
+    ssx = dials_data("cunir_serial_processed", pathlib=True)
+    expts = str(ssx / "imported_no_ref_5.expt")
+    result = subprocess.run(
+        [shutil.which("dxtbx.plot_detector_models"), expts, "pdf_file=plot_test.pdf"],
+        cwd=tmp_path,
+        capture_output=True,
+    )
+    assert not result.returncode and not result.stderr
+    assert os.path.exists(tmp_path / "plot_test.pdf")