EmonCMS – a powerful open-source web-app for processing, logging and visualising energy, temperature and other environmental data
It will run on a Raspberry Pi (or similar) computer to gather data, and can be viewed on an iOS or Android app. Dashboards can also be published for public viewing. Emoncms logs its data from any MQTT enabled home automation device, so it can compliment Home Assistant quite nicely (and others like OpenHAB, Node-RED, OpenEVSE).
There is a video at https://www.youtube.com/watch?v=eqN9rc9VnqI showing how dashboards can easily be created.
See the source code and documentation at http://bit.ly/2FnQ6l0
