If y axis min and max are set automatically in highchart stocks, they are set to min and max of the data in the highlighted area.
Here is an example: https://chapkovski.github.io/anomaly_tr ... gnedId=123
In case the link does not work, here how it looks like:
https://imgur.com/a/TnOIXRY
But is there a way to automatically set y lims depending on the currently zoomed data + some margin (like double of current data min and data max)?