Hello @reportter, welcome to the Quick Sight community!
There may be a few issues here causing your data to not return as expected. When you say the cumulative amount, what calculation are you utilizing to show the cumulative total across the year?
Also, how are you grouping the color by field to split the cumulative amount by month? Depending on your data, it would be really beneficial in the debugging process, if you are able to create a demo version of this visual in Quick Sight Arena where I could test possible solutions directly in an analysis. Here is a blog with some information on Arena:
With some more information, I can help guide you further. Thank you!