Use Agg backend for mpl in CI

Add matplotlib.use('Agg') to the code examples, which is necessary when run in
an environment without X11 sever, such as the CI pipeline.

README.md

@@ -98,7 +98,7 @@ Numpy arrays overload the typical arithmetic operations, such that the above
 statement benefits from numpys efficient, vectorized (i.e. performing the same
 operation on may values) implementation. You should always think about a way to
 use such vectorized statements, and try to avoid manually looping over all the
-values. Using a python loop to run over 10^3 values is probably fine, but you
+values. Using a python loop to run over $`10^3`$ values is probably fine, but you
 don't want to wait for a python loop iterating over 10^6 or 10^9 values.
 
 Finally add a print statement to check that all the caluclations are as expected. 
@@ -127,11 +127,19 @@ the cropped parabola f(x), which is limited y=4 for x>=2.
 ```math
     f(x) = \left\{\begin{array}{lr}
               x^2, & \text{for } x < 2\\
-              4, & \text{for } 2 \leq x\\
-            \end{array}\right\} = xy
+              4, & \text{for } 2 \leq x
+            \end{array}\right\.
 ```
 
 Create the file `func_plot.py` and add the following lines.
+
+<!-- Add additional files for non-X11 environment in CI -->
+<!-- write func_plot.py
+```python
+import matplotlib
+matplotlib.use('Agg')
+```
+-->
 <!-- write func_plot.py -->
 ```python
 import numpy as np