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This use case might be difficult with the current infrastructure since tidbyt cannot access local network resources.
However maybe this can be accomplished 2 ways.
Today: you can have a pixlet server running on your pi, and on an interval, serve the rendered pixlet containing the metrics you desire using the API. This would work with current tidbyt today.
Future: this can be tricky, but perhaps an app can be built and ran on the tidbyt, and the user can specify a data source URL. Then you would need some method to make the pi data available externally, like a REST Api. This app could then call the REST api on your box to get the metrics in a json, then render it for displaying. This would require port forwarding and making your Pi visible to the internet, which isn’t necessarily ideal.
Hope this is good food for thought