can't quite get this to run against my vimwiki

Hello!

I'm hacking around with some ideas for navigating/augmenting the stuff I've got in vimwiki, and it's sort of clear I need to generate some data structures for representing the link graph. I was googling around to see if anyone had done that kind of thing and came across your portfolio site just now.

This looks like a neat tool, but I haven't quite managed to get it to run. So far I've done this on my Debian Buster system:

Dependency installs:

```sh
pip3 install networkx
pip3 install numpy
pip3 install matplotlib
pip3 install ipdb
```

Attempted runs - first in the test directory then inside my wiki directory:

```
0:30:35 brennen@inertia:~/code/vimwiki_link_network (master *=) ☆ python3 vimwiki_link_network.py index Graph --save
/home/brennen/.local/lib/python3.7/site-packages/matplotlib/backends/backend_gtk3.py:195: Warning: Source ID 8 was not found
when attempting to remove it
GLib.source_remove(self._idle_draw_id)
```

```
0:51:26 brennen@inertia:~/notes/vimwiki (master *%=) ✿ python3 vimwiki_link_network.py index.wiki DiGraph --save
/home/brennen/.local/lib/python3.7/site-packages/matplotlib/backends/backend_gtk3.py:195: Warning: Source ID 8 was not found when attempting to remove it
GLib.source_remove(self._idle_draw_id)
```

Doesn't seem to make much difference if I use `Graph` or some other type. In both cases I get an empty PNG like this one:


If you notice anything I'm obviously doing wrong here, pointers would be most appreciated. Regardless, thanks for publishing this.

Start with the commands shown for vimwiki_link_network.py, using the test directory and the wiki directory inputs. Inspect how the tool handles the Vimwiki input and graph type, then verify that the saved PNG contains the expected link graph rather than being empty.

Written by the indexing model from the issue text.