Invalid display variable: Matplotlib issue with Jupyter

I was getting the following error when running the **mnist_demo notebook.**

``` python
136 display = os.environ.get('DISPLAY')
137 if display is None or not re.search(':\d', display):
--> 138 raise RuntimeError('Invalid DISPLAY variable')
139
140 qApp = QtWidgets.QApplication([str(" ")])

RuntimeError: Invalid DISPLAY variable
```

The solution is to use magic:
`%matplotlib inline`
