'plotcount.py isles.dat show' doesn't open a window 

As mentioned [here](https://github.com/hpc-carpentry/hpc-python/blob/gh-pages/_episodes/11-snakemake-intro.md), the call to 
```
$ python3 plotcount.py isles.dat show
```
should open a window to produce a plot. On my system:
```
Python 3.7.5 (default, Oct 17 2019, 12:09:47) 
[GCC 9.2.1 20190827 (Red Hat 9.2.1-1)] on linux
```
Nothing happens.

