I'd like to generate a "burn down" chart, which is a simple forecasting tool for agile software practices.  The first problem I encounter is that my actual data is a sum that is a single point, say value 400.  Now, I'd like to forecast the "Burn down" by applying a parameter (called "velocity") to the sum, iteratively, until it reaches zero.

So the resulting visualization would have 6 columns: 400, 320, 240, 260, 80, 0.

Ideally now I would label all those columns with dates.

Super duper ideally I would be able to start the process over again with the next sum in my table, continuing the date series from where the last one reached zero.

Is there such a function that can create new buckets based on the data?

Or can I use R to do it?  It does not seem I can continue to call the R functions recursively, nor can they return a set.

The result would be something like the below.  I can generate these data sets manually, but itis a pain. I'd rather have Tableau generate the forecasting data.