fix matplotlib import

hysop/backend/device/kernel_autotuner_statistics.py

 
 import sys
 import numpy as np
-from matplotlib import pyplot as plt
 from hysop.tools.types import check_instance
 
 class AutotunedKernelStatistics(dict):
@@ -56,6 +55,7 @@ class AutotunedKernelStatistics(dict):
         self.plot_histogram()
 
     def plot_histogram(self):
+        from matplotlib import pyplot as plt
         run_times = self.run_times.copy()
         for unit in ('ns', 'us', 'ms', 's'):
             if run_times.min() < 1e2: