Chart clear set a earliest_time_to_chart marker

I would suggest that in instead of clearing the data (a user settable) “marker” (earlist_time_to_chart) be set which Station preserves. Any data before that “marker” is not displayed in the chart. The user could “unclear” the Markey by nullifying it as well as setting it to a different value. This would be very helpful if the tag is moved to a different , unrelated location. This would be necessary when min, average, max are implemented.

This would also resolve the problem on clearing the data on the tag and then having the data repopulated by the gateway acquired data.