Prevent overwrite of Agg setup

Prevent the second operation in func_plot.py to overwrite the first one, which
set up the Agg backend.

README.md

@@ -140,7 +140,7 @@ import matplotlib
 matplotlib.use('Agg')
 ```
 -->
-<!-- write func_plot.py -->
+<!-- append func_plot.py -->
 ```python
 import numpy as np
 import matplotlib.pyplot as plt