Are you asking how to script a temporary data holder for the past 8 hours without using historization?
I think you could insert data to the historian, and remove it.
I don't think you will want to.
If you historize it:
Ad Hoc trends from Ignition exchange is what I am using at the moment in some cases.
PTurmel has a really nice viewer project as well.
There is a realtime mode on the power table.
I think you want the ad hoc trend though.
I also use a timeserieschart with historical setup like:
now(0) and aritehmetic (now(0), -8,'hour)
Maybe I don't know what a realtime tag is though.
A tag has to be historized, or have it's past recorded into something via transaction or script to pull up that data.