Situation: history graph card plotting the state of a rarely-changing entity. In my case, it’s the fan speed of the network switch at my church which is typically only at 70 or 90 arbitrary units.
If the state hasn’t changed for a while (the switch fan tends to stay on 70 for the whole winter, for example), the card doesn’t display at all.
this is good finding. I was thinking about it recently when adding trv’s target temperature to graphs.
In most cases values recorded by HA change often enough to make most recent data remaining in history
In case of rarely changing values it may happen that purging process wipes all data leaving nothing to display.
There might be several solutions to that. For example leaving most recent data point in db regardless its age.
But definitively it should be considered as important flaw.