I’m glad to know you were able to make it work 
!
Coda is pretty smart and interpret a lot of things all by itself but Copy/Pasting can, sometimes, lead to unfortunate errors 
.
If you just deleted the very first Month in the Filter() formula, Coda would have normally suggested you the “right” one (i.e.: the one where CurrentValue.Month is implied 
)
And you can always click on a value you use in a formula to get more info about it in the formula editor …
(See the highlighted Month in the screenshot below)
As for this :
It is sadly not natively feasible as Coda doesn’t support combined type of charts at the moment 
.
Maybe there’s a pack in the Gallery that could do it 
, I don’t know (sorry 
)…
Another possibility, a bit complex, would be to look at SVGs … or embed such a chart from GSheets (for example)