In QV, we can set it to display a scroll bar, but default it to display the last 26 values:
This will accumulate it correctly, while still only showing the last 26 values on default (giving the user the ability to go back further in time if they so wish). See the attached for full example.
If you don't want to go this route, I believe you will need to calculate the rolling average in your load script instead of in the chart itself.
example.qvw 148.8 K