Call matplotlib.use('Agg') in json_to_dataset.py

.travis.yml

@@ -11,7 +11,6 @@ matrix:
         - SYSTEM_PYTHON=/usr/bin/python
         - PIP=pip
         - PYTHON=python
-        - MPLBACKEND=Agg
       addons:
         apt:
           packages:

labelme/cli/json_to_dataset.py

-#!/usr/bin/env python
-
 import argparse
 import json
 import os
 import os.path as osp
 import warnings
 
+import matplotlib
+matplotlib.use('Agg')
+
 import numpy as np
 import PIL.Image
 import yaml