improve based on Paul's comments

openmc/data/multipole.py

@@ -8,8 +8,6 @@ import h5py
 import pickle
 import numpy as np
 from scipy.signal import find_peaks
-import matplotlib
-matplotlib.use("agg")
 import matplotlib.pyplot as plt
 
 import openmc.checkvalue as cv
@@ -646,7 +644,8 @@ def _windowing(mp_data, n_cf, rtol=1e-3, atol=1e-5, n_win=None, spacing=None,
         energy_sqrt = np.linspace(np.sqrt(e_start), np.sqrt(e_end), n_points)
         energy = energy_sqrt**2
 
-        # reference xs from multipole form
+        # reference xs from multipole form, note the residue terms in the
+        # multipole and vector fitting representations differ by a 1j
         xs_ref = vf.evaluate(energy_sqrt, poles, residues*1j) / energy
 
         # curve fit matrix

tests/unit_tests/test_data_multipole.py

@@ -5,14 +5,6 @@ import numpy as np
 import pytest
 import openmc.data
 
-no_vectfit = False
-try:
-    import vectfit as vf
-except:
-    no_vectfit = True
-
-vf_only = pytest.mark.skipif(no_vectfit, reason="vectfit package not installed")
-
 
 @pytest.fixture(scope='module')
 def u235():
@@ -56,16 +48,16 @@ def test_export_to_hdf5(tmpdir, u235):
     assert os.path.exists(filename)
 
 
-@vf_only
 def test_from_endf():
+    pytest.importerskip('vectfit')
     endf_data = os.environ['OPENMC_ENDF_DATA']
     endf_file = os.path.join(endf_data, 'neutrons', 'n-001_H_001.endf')
     return openmc.data.WindowedMultipole.from_endf(
             endf_file, wmp_options={"n_win": 400, "n_cf": 3})
 
 
-@vf_only
 def test_from_endf_search():
+    pytest.importerskip('vectfit')
     endf_data = os.environ['OPENMC_ENDF_DATA']
     endf_file = os.path.join(endf_data, 'neutrons', 'n-095_Am_244.endf')
     return openmc.data.WindowedMultipole.from_endf(