ok,
can you share the datapoint lists that do and don't work. How do you build them, is it different if you group by Day of Week immediately.
The results depend on things like sorting of your data points, how about that, do you see any differences ?
If not (i.e. on both occasions, the datapoints lists are EXACTLY the same, when it does and does not plot) then next thing to look at, is if you somehow build up your datapoint list without clearing it, having double data in there that somehow confuse the charting library, maybe...)
So again, I'd put a table with my datapoints in the UI to see what exact datapoint list goes with what exact chart result, then share some screenprints here.
Dorine