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like the bollinger band example here? https://observablehq.com/@observablehq/plot-window#bollingerBand

Not quite sure what @Joe Bruce is trying to accomplish. It seems like any solution would involve adding a constant for one windowY and subtracting a constant for another windowY. Using the bollingerBand seems a principled way of doing that and provides some useful statistical info. If you add and subtract the same constant, you can get two series, but you could accomplish the same thing just by increasing the strokeWidth. My quick experiments at https://observablehq.com/d/1b8cb8fba5a4f6d5

I was hoping to take the min value and max value within a time window to create a single band. bollinger is similar but not quite what I was going for.

Maybe try using “max” and “min” as the reducers for two windowY’s. I just tried that in https://observablehq.com/d/1b8cb8fba5a4f6d5