Bug 471402

These are the steps I went through. I guess there is a way to add a missing sensor more directly though, maybe through modifying a config file to add some fake sensor.
1. Add some sensors to some graphs on the History page
2. Install updates that remove those sensors
3. Go to edit the History page and see the "This page is missing some sensors and will not display correctly." message
4. Click "Fix"

In my case, I had some AMD GPU sensors change in a way that wasn't really a 1-1 mapping, so it didn't make sense to replace the existing sensors I had before. I would rather just delete the old ones, but there is no direct way to do this. I need to assign these to some sensor that exists before I can delete them.

I feel like one of the options to resolve a missing sensor should be to just delete it. Maybe add a button next to the "Click to select a sensor..." box, with a trashcan icon.