General global altitude field offset

[AndreasAntener]

• Either specify manually
• Or take starting altitude from some global alt field automatically

Then add offset to all fields we know they contain global altitude.
Should be a checkbox to enable/disable

[DrTon]

That would be good if we have log format with standard fields, but we have no one, each autopilot has own field names with different names and meanings and to do it correct way need to handle all cases (APM, PX4, PX5, etc...). So not sure that it's good idea to add new hack. Offset field is always available, moreover baro and GPS have large offset so to compare them user must adjust offset manually. Maybe it's better to:

• Provide some method in GUI for easy drag-n-drop moving of lines on the chart? That would be more useful maybe.
• For ANY line add checkbox "adjust vertical offset so line starts from 0 at start of log" and/or "adjust vertical offset so line starts from 0 at start of view", this would be useful not only for altitude but also for other cases, like integral of acceleration.
• Other ideas?