Add opt-in inactivity expiry for ui-chart series/topics

[omidteimoori]

Summary

• add an opt-in ui-chart setting to expire inactive series/topics after a configurable timeout
• remove only the inactive series from chart state so active series remain untouched
• preserve existing ui-chart behavior when the option is disabled

Refs issue #2090

[colinl]

Have you considered how this will work if the data points are added as a complete set of historical data from, for example, a database? Will all the legends suddenly disappear after the specified time?

[omidteimoori]

Yes. In the current implementation, inactivity is measured from the last time a series was received by the dashboard, not from the datapoint’s own x-value. So if a chart is populated from a one-off historical dataset and no further updates arrive, those series will expire after the configured timeout. That is intentional for the live-data use case this option targets.
For historical/static datasets, users can simply leave the option disabled, in which case ui-chart behaves exactly as it does today.

[colinl]

Would it not have been much simpler to limit it to timestamp x axes and use the timestamp of the latest sample?

The PR you have submitted is remarkably complex and will take a lot of time to review, I suspect you will find it difficult to find a developer to do that.