Allow tests that require GUI (matplotlib)

.travis.yml

@@ -24,6 +24,7 @@ env:
     - OPENMC_ENDF_DATA=$HOME/endf-b-vii.1
     - OPENMC_MULTIPOLE_LIBRARY=$HOME/multipole_lib
     - PATH=$PATH:$HOME/NJOY2016/build
+    - DISPLAY=:99.0
   matrix:
     - OMP=n MPI=n PHDF5=n
     - OMP=y MPI=n PHDF5=n

tools/ci/travis-before-script.sh

@@ -1,6 +1,10 @@
 #!/bin/bash
 set -ex
 
+# Allow tests that require GUI as described at:
+# https://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-xvfb-to-Run-Tests-That-Require-a-GUI
+sh -e /etc/init.d/xvfb start
+
 # Download NNDC HDF5 data
 if [[ ! -e $HOME/nndc_hdf5/cross_sections.xml ]]; then
     wget https://anl.box.com/shared/static/a6sw2cep34wlz6b9i9jwiotaqoayxcxt.xz -O - | tar -C $HOME -xvJ