Stacked column chart breakdown with a measure

Hi. I created a stacked column chart to show customer credit (in dollars) over time. On the y-axis are dollars, and on the x-axis is the year. Each bar is broken down by reason the customer was credited.

Here is a simplified example of the chart:

Here is the underlying data:

I would like to modify the column chart so that all the amounts for a given year are normalized by SUM(SALES) for that year. The result would be a chart similar to the one shown above, except with percent (of sales) on the y-axis.

I tried to do this by creating a measure that performs Credit/Sales. However, if I use this measure as the 'Values' field in my column chart, I lose the ability to break down the column by reason. In order to see any data I must remove the 'Legend' field. 

Is there a way to show Credit/Sales broken down by reason in my column chart? Thank you.