Find min/max chart timestamps. Fixes #308
Find min/max chart timestamps. Fixes #308

There is only one series per axis, so the 2/3/4th sensors come after the
1st and might have earlier dates.

MastodonC/kixi.hecuba,MastodonC/kixi.hecuba,MastodonC/kixi.hecuba,MastodonC/kixi.hecuba,MastodonC/kixi.hecuba