Comment 2 for bug 1957997

I've encountered this issue on a customer deployment. Had to blank the install_plugins config option and install the zips by hand:

juju config -m lma grafana --reset install_plugins
juju ssh -m lma grafana/leader "sudo apt install unzip;
cd /tmp;
https_proxy=http://10.49.12.10:8000 wget https://storage.googleapis.com/plugins-community/vonage-status-panel/release/1.0.11/vonage-status-panel-1.0.11.zip;
https_proxy=http://10.49.12.10:8000 wget https://storage.googleapis.com/plugins-community/grafana-piechart-panel/release/1.6.2/grafana-piechart-panel-1.6.2.zip;
sudo unzip vonage-status-panel-1.0.11.zip -d /var/snap/grafana/common/data/plugins;
sudo unzip grafana-piechart-panel-1.6.2.zip -d /var/snap/grafana/common/data/plugins;
sudo sudo snap restart grafana"