You need the last grafana build to enable plugin support. You can get it here : http://grafana.org/download/builds.html Unfortunately, the docker image, the package available in the repos don't have plugin support enabled.
Either clone this repo into your grafana plugins directory (default /var/lib/grafana/plugins if your installing grafana with package). Restart grafana-server and the plugin should be automatically detected and used.
git clone https://github.com/creative-area/grafana-panel-plugin-horizon.git creative-area-horizon-panel
sudo service grafana-server restart
You can change plugins directory with GF_PATHS_PLUGINS
environment variable when running Grafana.
# with service
GF_PATHS_PLUGINS=/path/to/grafana/plugins sudo service grafana-server restart
# in development mode
GF_PATHS_PLUGINS=/path/to/grafana/plugins ./bin/grafana-server
Then edit your grafana.ini config file (Default location is at /etc/grafana/grafana.ini) and add this:
[plugin.horizon]
path = /home/your/clone/dir/grafana-panel-plugin-horizon
Note that if you clone it into the grafana plugins directory you do not need to add the above config option. That is only if you want to place the plugin in a directory outside the standard plugins directory. Be aware that grafana-server needs read access to the directory.