The desklet is based on Network usage monitor applet from clem.
How it does it:
The vnstat daemon runs in the background and collects info about your Internet usage.
The applet detect which device youre currently using, and simply export a graph using vnstati.
What you need for it to work:
- To install vnstat
- To install vnstati
- To have the vnstat daemon running
Note: In Linux Mint, you can simply run
apt install vnstati and that will take care of everything. In other distributions it might depend on the way things are packaged but its likely to be similar.