changing source data for "Lines on 2 Axes" graph

I can't seem to figure out how to modify the source data for a custom graph (specifically, the "Lines on 2 Axes" graph). I have named several ranges in the code up to this point, but I don't think I can use them because the source data is in the form of series collections (?), so I can't just use the SetSourceData command...right? Anyway, if someone could please explain this to me and propose a solution, I would be much appreciative. I recorded a macro to see if I could modify it accordingly, but I don't know enough about VB to do so.

ActiveChart.SeriesCollection(1).XValues = "=Output!R17C3:R18C3"
ActiveChart.SeriesCollection(1).Values = "=Output!R17C7:R18C7"
ActiveChart.SeriesCollection(2).XValues = "=Output!R17C3:R18C3"
ActiveChart.SeriesCollection(2).Values = "=Output!R17C6:R18C6"
ActiveChart.SeriesCollection(3).Values = "=Blend!R3C15:R6C15"

I can't figure out where the ranges are coming from here. They're obviously not cell references in the traditional sense, which is why I figure I can't use the ranges to which I've already assigned names. thanks in advance.