Hi Alec,
Hope you are doing good. I am facing the same kind of issue.
I have power chart binding to the historical tag. I have set default time range for 1 hour Realtime. My Y axis value was oscillating according to the value reading in the tag.
I set the Min and Max value and off the auto range to false, still value curve was oscillating when time changes . Can you please provide the solution for this issue.