Charting cumulative sum and residual

I've got a years worth of sales data, by day. I've also got an annual sales target.

I'm trying to duplicate a Cognos report that shows 'sales remaining to goal' stacked over 'sales to date' for the year. Sort of looks like a rectangle with a diagonal split rising from left to right (party per bend sinister if you know your heraldry - ).

The first part is fairly easy - a cumulative sum of sales by day. Although I can't find a field to set the custom field name, so I'm left with an ugly axis title 'Cumulative sum of DAY_SALES'.

The second part is more difficult: the total - the first part. The cumulative sum only exists in the Kibana report, so I can't use it in a scripted field or the like.

I could fake by using non-stacked area graphs, but then the 'remaining' field in the inspect report would be wrong/missing.

Given the cumulative count resets to 0 if I change the filters to look at an area of the graph within the year, I'm starting to think that Kibana may not be the place to calculate these values.

Could it be done with a transform? How horrible would the performance cost be, given I'd want the transform running continuously to integrate new sales data every day?

Could you please attach screenshots of what you want to achieve, vs. what you're currently getting?