Personally, I think that this would be better as a widget, but it would require the guys at Garmin to provide a hook to execute a small bit of code periodically to work well. If that functionality were available, you'd have a pressure chart that looked like this, but it would automatically update. On some devices you can access the watch face and widget list while in an activity. I believe that the vivoactive requires you to long press on the screen, and the fenix3 requires you to hold the down button.
Until that happens, I think you might be best off to ask babysimon_uk to do it. He's got the code to do the graph as a data field already. Theoretically this is just a few lines of code for him.
Elevation is not the same as barometric pressure. I suggested that the best solution would be to ask the author of that app to make a barometer Field, not realizing that the barometric pressure is only in the Sensor module, and a data field does not have access.
